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.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273809 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f