MFC r273711:
Allocate buffer for READ BUFFER/WRITE BUFFER commands on demand.

These commands are rare, but consume additional 256KB RAM per LUN.
MFC r273730, r273731:
Reduce code duplication around Write Exclusive persistent reservation.

While there, allow some more commands to pass persistent reservation.
MFC r273809:
Implement better handling for ENOSPC error for both CTL and CAM.

This makes VMWare VAAI Thin Provisioning Stun primitive activate, pausing
the virtual machine, when backing storage (ZFS pool) is getting overflowed.