xfs_repair: properly detect reserved attribute names

xfs_repair: properly detect reserved attribute names

This function in xfs_repair tries to make sure that if an attr
name reserved for acls exists in the root namespace, then its
value is a valid acl.

However, because it only compares up to the length of the
reserved name, superstrings may match and cause false positive
xfs_repair errors.

Ensure that both the length and the content match before
flagging it as an error.

Spotted-by: Zach Brown <zab@zabbo.net>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>