Add to CTL support for threshold notifications for file-backed LUNs.

Previously it was supported only for ZVOL-backed LUNs, but now should work
for file-backed LUNs too. Used value in this case is a space occupied by
the backing file, while available value is an available space on file
system. Pool thresholds are still not implemented in this case.

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