NFSD: Constify @fh argument of knfsd_fh_hash()

[ Upstream commit 1736aec8 ]

Enable knfsd_fh_hash() to be invoked in functions where the
filehandle pointer is a const.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>