[CIFS] cifs: remove pointless lock and unlock of GlobalMid_Lock in header_assemble

We lock GlobalMid_Lock in header_assemble and then immediately unlock it
again without doing anything. Not sure what this was intended to do, but
remove it.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>