nfs: remove nfs_compat_user_ino64() and deprecate enable_ino64

nfs: remove nfs_compat_user_ino64() and deprecate enable_ino64

Now that inode->i_ino stores the full 64-bit NFS fileid, the
nfs_compat_user_ino64() function is no longer needed.
generic_fillattr() already copies inode->i_ino into stat->ino, so the
explicit override in nfs_getattr() is also redundant.

Also remove the now-unused nfs_fileid_to_ino_t() and
nfs_fattr_to_ino_t() helper functions that were used to XOR-fold
64-bit fileids into the old unsigned long i_ino.

Keep the enable_ino64 module parameter as a deprecated stub that
accepts but ignores the value, logging a notice when set. This avoids
breaking existing configurations that pass nfs.enable_ino64 on the
kernel command line or in modprobe.d.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>