NFSD: Optimize nfsd4_encode_operation()

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, and here, it is unnecessary because the
data item is fixed in size, the buffer destination address is
always word-aligned, and the destination location is already in
@p.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -5373,8 +5373,7 @@ nfsd4_encode_operation(struct nfsd4_compoundres *resp, struct nfsd4_op *op)
 						so->so_replay.rp_buf, len);
 	}
 status:
-	/* Note that op->status is already in network byte order: */
-	write_bytes_to_xdr_buf(xdr->buf, post_err_offset - 4, &op->status, 4);
+	*p = op->status;
 }
 
 /*