NFSD: Don't apply NFS version-specific behavior to LOCALIO requests

LOCALIO serves NFS clients of every version through one entry point,
so no protocol version is associated with such a request.
nfsd_set_fh_dentry() selects version-specific behavior anyway: its
switch keys off fh_maxsize, and nfsd_open_local_fh() passes
NFS4_FHSIZE because that is the size of the buffer it copies into, so
LOCALIO lands in the NFSv4 arm. fh_getattr() keys off fh_maxsize too
and does run on a LOCALIO open, adding STATX_BTIME and
STATX_CHANGE_COOKIE to the mask it requests: work on filesystems that
compute them for a caller that never reads them.

nfsd_open_local_fh() only verifies a handle it received, so it has no
maximum size to state. Pass NFSD_FHSIZE_UNSPEC as nlm_fopen() already
does, which selects the switch arm that applies no version-specific
behavior, and state the bound on the copy out of struct nfs_fh as
NFS_MAXFHSIZE.

Suggested-by: NeilBrown <neil@brown.name>
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Link: https://patch.msgid.link/20260728165911.462534-6-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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