New marshalling helpers - ncp_add_be16() and ncp_reply_be16().

Encode and decode a big-endian 16bit field resp.

A bunch of places in ncplib_kernel.c switched to usign these.
Signed-off-by: Al Viro <viro@parcelfarce.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>