nfs4.2: add UNCACHEABLE_FILE_DATA attribute support

Recognize the NFSv4.2 per-file UNCACHEABLE_FILE_DATA attribute (attr 87,
draft-ietf-nfsv4-uncacheable-files): decode it via GETATTR, track per-
exported-filesystem support, and record on the inode whether a regular
file's data must not be cached.  Acting on the attribute (opening such
files O_DIRECT) is done by a subsequent change.

If the NFSv4 server reports a regular file's UNCACHEABLE_FILE_DATA as
true, it indicates the file's data must not be cached; the client records
this in NFS_I(inode)->uncacheable_file_data for use by the I/O paths.

The UNCACHEABLE_FILE_DATA attribute applies only to regular files
(NF4REG); per the draft a server MUST reject a query of it on any other
object type with NFS4ERR_INVAL.  A subsequent commit gates the client
accordingly.

Link: https://datatracker.ietf.org/doc/draft-ietf-nfsv4-uncacheable-files/

Signed-off-by: Tom Haynes <loghyr@hammerspace.com>
[snitzer: adapt Tom's original code focused on metadata for ABE]
Co-developed-by: Mike Snitzer <snitzer@hammerspace.com>
Signed-off-by: Mike Snitzer <snitzer@hammerspace.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

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