smb: client: fix filename matching of deferred files

Fix the following case where the client would end up closing both
deferred files (foo.tmp & foo) after unlink(foo) due to strstr() call
in cifs_close_deferred_file_under_dentry():

  fd1 = openat(AT_FDCWD, "foo", O_WRONLY|O_CREAT|O_TRUNC, 0666);
  fd2 = openat(AT_FDCWD, "foo.tmp", O_WRONLY|O_CREAT|O_TRUNC, 0666);
  close(fd1);
  close(fd2);
  unlink("foo");

Fixes: e3fc0656 ("cifs: Deferred close performance improvements")
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Cc: Frank Sorenson <sorenson@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>