switch l2cap ->memcpy_fromiovec() to msghdr

it'll die soon enough - now that kvec-backed iov_iter works regardless
of set_fs(), both instances will become copy_from_iter() as soon as
we introduce ->msg_iter...

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

diff --cc include/net/bluetooth/l2cap.h
Simple merge

diff --cc net/bluetooth/l2cap_core.c
Simple merge

diff --cc net/bluetooth/l2cap_sock.c
Simple merge