[CIFS] In SendReceive, move consistency check out of the mutexed region

inbuf->smb_buf_length does not change in in wait_for_free_request() or in
allocate_mid(), so we can check it early.

Signed-off-by: Volker Lendecke <vl@samba.org>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>