cifs: release lock earlier in dequeue_mid error case

In dequeue_mid we can log an error while holding a spinlock,
GlobalMid_Lock.  Coverity notes that the error logging
also grabs a lock so it is cleaner (and a bit safer) to
release the GlobalMid_Lock before logging the warning.

Addresses-Coverity: 1507573 ("Thread deadlock")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>