new helper: msg_data_left()

convert open-coded instances

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc crypto/algif_hash.c
Simple merge

diff --cc crypto/algif_skcipher.c
Simple merge

diff --cc drivers/vhost/net.c
Simple merge

diff --cc include/linux/socket.h
Simple merge

diff --cc net/core/datagram.c
Simple merge

diff --cc net/ipv4/tcp.c
Simple merge

diff --cc net/rxrpc/ar-output.c
Simple merge

diff --cc net/socket.c
Simple merge