nfsd: Report export case-folding via NFSv3 PATHCONF

nfsd: Report export case-folding via NFSv3 PATHCONF

The hard-coded MSDOS_SUPER_MAGIC check in nfsd3_proc_pathconf()
only recognizes FAT filesystems as case-insensitive. Modern
filesystems like F2FS, exFAT, and CIFS support case-insensitive
directories, but NFSv3 clients cannot discover this capability.

Query the export's actual case behavior through ->fileattr_get
instead. This allows NFSv3 clients to correctly handle case
sensitivity for any filesystem that implements the fileattr
interface. Filesystems without ->fileattr_get continue to report
the default POSIX behavior (case-sensitive, case-preserving).

This change depends on the earlier "fat: Implement fileattr_get
for case sensitivity" patch in this series, which ensures FAT
filesystems report their case behavior correctly via the
fileattr interface.

Case-folding is a per-directory property, so
nfsd_get_case_info() queries the parent dentry for
non-directory filehandles. Three inherent corner cases follow:
a single-file export's parent lies outside the exported
subtree, so the LSM hook evaluates against an unexported
directory; a disconnected dentry from fh_verify() has
d_parent == itself, so the file's own attributes are reported
until the dentry connects; and a hardlinked file resolves
through the alias the dcache currently holds, so when the
inode is linked into both case-folded and case-sensitive
directories the reported value tracks whichever parent is
active. These limitations are not addressable without
redefining the protocol attribute as per-parent rather than
per-object.

RFC 1813 restricts PATHCONF errors to NFS3ERR_STALE,
NFS3ERR_BADHANDLE, and NFS3ERR_SERVERFAULT. When an LSM hook
denies the case-folding query on the parent, NFS3ERR_STALE is
the only correct mapping: NFS3ERR_SERVERFAULT misrepresents a
working server as broken, and NFS3ERR_BADHANDLE implies a
decoding failure that did not occur. A client purging the
filehandle on receipt is the desired outcome, since the server
has refused to read attributes through it. Substituting POSIX
defaults instead would let the same handle report
casefold=false now and casefold=true once policy permits,
opening a silent name-collision window on case-insensitive
exports.

Reviewed-by: Roland Mainz <roland.mainz@nrubsig.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260507-case-sensitivity-v14-13-e62cc8200435@oracle.com
Signed-off-by: Christian Brauner <brauner@kernel.org>