lib: darr: fix bug with nested macro use

- WHen declaring macro scoped variables, can run into problem if the macro
variable passed in has the same name as the new variable introduced in the inner
scope. We don't get a warning and the uses will be wrong.

e.g.,

```

{
    int __len = 10;
    foo(__len); // => 10 and not 15 as we wanted.
}
```

Signed-off-by: Christian Hopps <[email protected]>

lib/darr.h

@@ -272,10 +272,10 @@ void *__darr_resize(void *a, uint count, size_t esize, struct memtype *mt);
  */
 #define darr_ensure_avail_mt(A, S, MT)                                         \
 	({                                                                     \
-		ssize_t need = (ssize_t)(S) -                                  \
-			       (ssize_t)(darr_cap(A) - darr_len(A));           \
-		if (need > 0)                                                  \
-			_darr_resize_mt((A), darr_cap(A) + need, MT);          \
+		ssize_t __dea_need = (ssize_t)(S) -                            \
+				     (ssize_t)(darr_cap(A) - darr_len(A));     \
+		if (__dea_need > 0)                                            \
+			_darr_resize_mt((A), darr_cap(A) + __dea_need, MT);    \
 		(A);                                                           \
 	})
 #define darr_ensure_avail(A, S) darr_ensure_avail_mt(A, S, MTYPE_DARR)
@@ -301,9 +301,9 @@ void *__darr_resize(void *a, uint count, size_t esize, struct memtype *mt);
 #define darr_ensure_cap_mt(A, C, MT)                                           \
 	({                                                                     \
 		/* Cast to avoid warning when C == 0 */                        \
-		uint _c = (C) > 0 ? (C) : 1;                                   \
-		if ((size_t)darr_cap(A) < _c)                                  \
-			_darr_resize_mt((A), _c, MT);                          \
+		uint __dec_c = (C) > 0 ? (C) : 1;                              \
+		if ((size_t)darr_cap(A) < __dec_c)                             \
+			_darr_resize_mt((A), __dec_c, MT);                     \
 		(A);                                                           \
 	})
 #define darr_ensure_cap(A, C) darr_ensure_cap_mt(A, C, MTYPE_DARR)
@@ -428,12 +428,12 @@ void *__darr_resize(void *a, uint count, size_t esize, struct memtype *mt);
 
 #define _darr_append_n(A, N, Z, MT)                                            \
 	({                                                                     \
-		uint __len = darr_len(A);                                      \
-		darr_ensure_cap_mt(A, __len + (N), MT);                        \
-		_darr_len(A) = __len + (N);                                    \
+		uint __da_len = darr_len(A);                                   \
+		darr_ensure_cap_mt(A, __da_len + (N), MT);                     \
+		_darr_len(A) = __da_len + (N);                                 \
 		if (Z)                                                         \
-			memset(&(A)[__len], 0, (N)*_darr_esize(A));            \
-		&(A)[__len];                                                   \
+			memset(&(A)[__da_len], 0, (N)*_darr_esize(A));         \
+		&(A)[__da_len];                                                \
 	})
 /**
  * Extending the array's length by N.