Remove session locking from cfiscsi_pdu_update_cmdsn().

cs_cmdsn can be incremented with single atomic. expcmdsn/maxcmdsn set in
cfiscsi_pdu_prepare() based on cs_cmdsn are not required to be updated
synchronously, only monotonically, that is achieved with lock there.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.