NFSD: Remove nfsd_file::nf_hashval

[ Upstream commit f0743c2b ]

The value in this field can always be computed from nf_inode, thus
it is no longer used.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>