NFSD: Constify @fh argument of knfsd_fh_hash()

[ Upstream commit 1736aec82a15cb5d4b3bbe0b2fbae0ede66b1a1a ]

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>
Stable-dep-of: cd35b6cb4664 ("nfs: add missing selections of CONFIG_CRC32")
Signed-off-by: Sasha Levin <sashal@kernel.org>