NFS: Add support for fallocate(FALLOC_FL_ZERO_RANGE)

This implements a suggestion from Trond that we can mimic
FALLOC_FL_ZERO_RANGE by sending a compound that first does a DEALLOCATE
to punch a hole in a file, and then an ALLOCATE to fill the hole with
zeroes. There might technically be a race here, but once the DEALLOCATE
finishes any reads from the region would return zeroes anyway, so I
don't expect it to cause problems.

Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>