Fix fall-through warnings in sunrpc/xdr.c.

This patch fixes implicit-fallthrough warnings in sunrpc/xdr.c when
building with -Wextra.  A fall-through comment is added in three
places; in two other places, an existing comment is reworded so it
matches the default patterns used by -Wimplicit-fallthrough.

Tested for x86_64.

	* sunrpc/xdr.c (xdr_int): Add fall-through comment.
	(xdr_u_int): Likewise.
	(xdr_enum): Likewise.
	(xdr_bytes): Reword fall-through comment.
	(xdr_string): Likewise.