nfscl: Add a VOP_DEALLOCATE() for the NFSv4.2 client

This patch adds a VOP_DEALLOCATE() to the NFS client.
For NFSv4.2 servers that support the Deallocate operation,
it is used. Otherwise, it falls back on calling
vop_stddeallocate().

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31640