cifs: Fix xattr name checks

Use strcmp(str, name) instead of strncmp(str, name, strlen(name)) for
checking if str and name are the same (as opposed to name being a prefix
of str) in the gexattr and setxattr inode operations.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>