Fix errorlint warnings

Errors returned from x/sys/unix are bare, so it's OK to compare
directly.

Fixes warnings like this one:

> mount_unix.go:26:19: comparing with == will fail on wrapped errors. Use errors.Is to check for a specific error (errorlint)
> 	if err == nil || err == unix.EINVAL {
> 	                 ^

Signed-off-by: Kir Kolyshkin <[email protected]>

mount/mount_unix.go

@@ -23,7 +23,7 @@ func Mount(device, target, mType, options string) error {
 // a normal unmount. If target is not a mount point, no error is returned.
 func Unmount(target string) error {
 	err := unix.Unmount(target, mntDetach)
-	if err == nil || err == unix.EINVAL {
+	if err == nil || err == unix.EINVAL { //nolint:errorlint // unix errors are bare
 		// Ignore "not mounted" error here. Note the same error
 		// can be returned if flags are invalid, so this code
 		// assumes that the flags value is always correct.

mountinfo/mounted_linux.go

@@ -23,7 +23,7 @@ func mountedByOpenat2(path string) (bool, error) {
 		Resolve: unix.RESOLVE_NO_XDEV,
 	})
 	_ = unix.Close(dirfd)
-	switch err {
+	switch err { //nolint:errorlint // unix errors are bare
 	case nil: // definitely not a mount
 		_ = unix.Close(fd)
 		return false, nil