NFSD: Use xdrgen XDR functions for NFSv2 SYMLINK procedure

A SYMLINK request's target pathname can extend from the head of the
receive buffer into its page data; the decoder reassembles it from a
head segment and a page via svc_fill_symlink_pathname(). Represent
the pathname in place rather than materializing it inline: the
"pages" directive expresses it as a struct xdr_buf and decodes it
with svcxdr_decode_opaque_payload(), which subsegments the source
stream instead of pulling the string inline through the bounded xdr
scratch buffer.

Annotate the to member of symlinkargs with "pragma pages" and
replace the NFSPROC_SYMLINK entry in nfsd_procedures2 with the
generated nfs_svc_decode_symlinkargs and nfs_svc_encode_nfsstat. A
wrapper structure bridges the generated symlinkargs type and the
svc_fh and iattr representations the NFSD VFS layer still uses.
nfsd_proc_symlink() copies the pathname out of the xdr_buf with
read_bytes_from_xdr_buf(), so a target that lands in the receive
buffer's tail, or straddles its pages and tail, is handled the same
way as one in the head.

The pc_argzero field is now set to zero for the NFSv2 SYMLINK
procedure. The generated decoder initializes every argument in the
argp->xdrgen field, making the early defensive memset unnecessary.

The hand-coded nfsd_proc_symlink() accepts a target of any length
from the decoder and answers one longer than NFS_MAXPATHLEN with
NFSERR_NAMETOOLONG. The generated decoder enforces the path typedef's
bound itself, so such a request now fails decoding and nfsd_dispatch()
returns RPC GARBAGE_ARGS instead. The Linux NFS client sends targets
up to PAGE_SIZE on an NFSv2 mount, and in that case it reports EIO to
the application rather than ENAMETOOLONG.

nfssvc_decode_symlinkargs() no longer has any callers, so it is
removed.

Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260916162859.2051-23-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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