CIFS: Fix VFS lock usage for oplocked files

We can deadlock if we have a write oplock and two processes
use the same file handle. In this case the first process can't
unlock its lock if another process blocked on the lock in the
same time.

Fix this by removing lock_mutex protection from waiting on a
blocked lock and protect only posix_lock_file call.

Cc: stable@kernel.org
Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>