NFSD: Optimize nfsd4_encode_fattr()

write_bytes_to_xdr_buf() is a generic way to place a variable-length
data item in an already-reserved spot in the encoding buffer.

However, it is costly. In nfsd4_encode_fattr(), it is unnecessary
because the data item is fixed in size and the buffer destination
address is always word-aligned.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -2828,10 +2828,9 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
 	struct kstat stat;
 	struct svc_fh *tempfh = NULL;
 	struct kstatfs statfs;
-	__be32 *p;
+	__be32 *p, *attrlen_p;
 	int starting_len = xdr->buf->len;
 	int attrlen_offset;
-	__be32 attrlen;
 	u32 dummy;
 	u64 dummy64;
 	u32 rdattr_err = 0;
@@ -2919,10 +2918,9 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
 		goto out;
 
 	attrlen_offset = xdr->buf->len;
-	p = xdr_reserve_space(xdr, 4);
-	if (!p)
+	attrlen_p = xdr_reserve_space(xdr, XDR_UNIT);
+	if (!attrlen_p)
 		goto out_resource;
-	p++;                /* to be backfilled later */
 
 	if (bmval0 & FATTR4_WORD0_SUPPORTED_ATTRS) {
 		u32 supp[3];
@@ -3344,8 +3342,7 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
 		*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);
+	*attrlen_p = cpu_to_be32(xdr->buf->len - attrlen_offset - XDR_UNIT);
 	status = nfs_ok;
 
 out: