Remove FirstBurstLength limit for software iSCSI.

For hardware offload solicited data may potentially be handled more
efficiently than unsolicited due to direct data placement. Or there
can be some unsolicited write buffering limitations. It may create
situations where FirstBurstLength limit is really useful.

Software driver though has no those factors, having to do memcopy()
any way and having no so hard limit on the temporary storage. Same
time more active use of unsolicited transfers allows to avoid some
of Ready To Transfer (R2T) PDU round-trip times and processing.

This change effectively doubles from 64KB to 128KB the maximum size
of write command that can be transferred within one link RTT. Tests
of (64KB, 128KB] QD1 writes mixed with simultaneous QD8 reads over
the same connection, increasing RTT, shows almost double write speed
and half latency, while we should be able to afford few megabytes of
RAM for additional buffering on a target these days.

Sponsored by: iXsystems, Inc.

(cherry picked from commit ff751ee05c939eceab25c26ad60b1d56f989aec9)