NFSD: Use xdrgen XDR functions for NFSv2 GETATTR procedure

Replace the NFSPROC_GETATTR entry in the nfsd_procedures2 array
with an entry that dispatches the xdrgen-generated
nfs_svc_decode_fhandle and nfs_svc_encode_attrstat. Wrapper
structures bridge the generated xdrgen types and the legacy
svc_fh and kstat representations the NFSD VFS layer still uses. A
few helper functions are introduced which will be used here and
in subsequent patches.

The pc_argzero field is now set to zero for the NFSv2 GETATTR
procedure. The xdrgen decoders are trusted to initialize all
arguments in the argp->xdrgen field, making the early defensive
memset unnecessary. The remaining argp fields are cleared as
needed.

This refactor replaces the use of svcxdr_encode_fattr(), so the
reference to the file handle can be released directly by
nfsd_proc_getattr(). A separate ->pc_release callback is thus no
longer needed. This makes it straightforward to confirm that the
file handle is properly released during every error flow.

The new nfsd_mode_to_ftype() utility no longer maps socket and
FIFO devices to non-standard values: NFSOCK=6 which is outside
the ftype enum, and NFCHR for FIFOs which is semantically wrong.
The Linux NFS client implementation does not seem to use or
depend on these non-standard values. The new utility function
returns NFNON for both, which is the RFC-compliant behavior.

Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260916162859.2051-10-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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