nfsd: Add support for the NFSv4.2 Deallocate operation

The recently added VOP_DEALLOCATE(9) VOP call allows
implementation of the Deallocate NFSv4.2 operation.

Since the Deallocate operation is a single succeed/fail
operation, the call to VOP_DEALLOCATE(9) loops so long
as progress is being made. It calls maybe_yield()
between loop iterations to allow other processes
to preempt it.

Where RFC 7862 underspecifies behaviour, the code
is written to be Linux NFSv4.2 server compatible.

Reviewed by: khng
Differential Revision: https://reviews.freebsd.org/D31624