[SUNRPC]: trivial endianness annotations

pure s/u32/__be32/

[AV: large part based on Alexey's patches]

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>