cifs: fix fh_mutex locking in cifs_reopen_file

Fixes a regression caused by commit a6ce4932

When this lock was converted to a mutex, the locks were turned into
unlocks and vice-versa.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Shirish Pargaonkar <shirishp@us.ibm.com>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>