nfsd: add fattr support for user extended attributes

Check if user extended attributes are supported for an inode,
and return the answer when being queried for file attributes.

An exported filesystem can now signal its RFC8276 user extended
attributes capability.

Signed-off-by: Frank van der Linden <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

Author: fllinden

fs/nfsd/nfs4xdr.c

@@ -3213,6 +3213,15 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
 	}
 #endif
 
+	if (bmval2 & FATTR4_WORD2_XATTR_SUPPORT) {
+		p = xdr_reserve_space(xdr, 4);
+		if (!p)
+			goto out_resource;
+		err = xattr_supported_namespace(d_inode(dentry),
+						XATTR_USER_PREFIX);
+		*p++ = cpu_to_be32(err == 0);
+	}
+
 	attrlen = htonl(xdr->buf->len - attrlen_offset - 4);
 	write_bytes_to_xdr_buf(xdr->buf, attrlen_offset, &attrlen, 4);
 	status = nfs_ok;