nfsd: clean up mounted_on_fileid handling

[ Upstream commit 6106d911 ]

We only need the inode number for this, not a full rack of attributes.
Rename this function make it take a pointer to a u64 instead of
struct kstat, and change it to just request STATX_INO.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
[ cel: renamed get_mounted_on_ino() ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>