Quiet ptrace error ESRCH in regsets_fetch_inferior_registers

When I run process-dies-while-detaching.exp with GDBserver, I see many
warnings printed by GDBserver,

ptrace(regsets_fetch_inferior_registers) PID=26183: No such process
ptrace(regsets_fetch_inferior_registers) PID=26183: No such process
ptrace(regsets_fetch_inferior_registers) PID=26184: No such process
ptrace(regsets_fetch_inferior_registers) PID=26184: No such process

regsets_fetch_inferior_registers is called when GDBserver resumes each
lwp.

 #2  0x0000000000428260 in regsets_fetch_inferior_registers (regsets_info=0x4690d0 <aarch64_regsets_info>, regcache=0x31832020)
    at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:5412
 #3  0x00000000004070e8 in get_thread_regcache (thread=0x31832940, fetch=fetch@entry=1) at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/regcache.c:58
 #4  0x0000000000429c40 in linux_resume_one_lwp_throw (info=<optimized out>, signal=0, step=0, lwp=0x31832830)
    at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:4463
 #5  linux_resume_one_lwp (lwp=0x31832830, step=<optimized out>, signal=<optimized out>, info=<optimized out>)
    at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:4573

The is the case that threads are disappeared when GDB/GDBserver resumes
them.  We check errno for ESRCH, and don't print error messages, like
what we are doing in regsets_store_inferior_registers.

gdb/gdbserver:

2016-08-04  Yao Qi  <[email protected]>

	* linux-low.c (regsets_fetch_inferior_registers): Check
	errno is ESRCH or not.

Author: Yao Qi

gdb/gdbserver/ChangeLog

@@ -1,3 +1,8 @@
+2016-08-04  Yao Qi  <[email protected]>
+
+	* linux-low.c (regsets_fetch_inferior_registers): Check
+	errno is ESRCH or not.
+
 2016-08-02  Yao Qi  <[email protected]>
 
 	* thread-db.c (struct thread_db) <td_ta_event_getmsg_p>: Remove.

gdb/gdbserver/linux-low.c

@@ -5405,6 +5405,12 @@ regsets_fetch_inferior_registers (struct regsets_info *regsets_info,
 		 not "active".  This can happen in normal operation,
 		 so suppress the warning in this case.  */
 	    }
+	  else if (errno == ESRCH)
+	    {
+	      /* At this point, ESRCH should mean the process is
+		 already gone, in which case we simply ignore attempts
+		 to read its registers.  */
+	    }
 	  else
 	    {
 	      char s[256];