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.

MFC after:	1 week
Sponsored by:	iXsystems, Inc.
Ticket:		#6468

(cherry picked from commit e1805defee550f3ab4638c681803c6b7dd62e266)