switch kernel_sendmsg() and kernel_recvmsg() to iov_iter_kvec()

For kernel_sendmsg() that eliminates the need to play with setfs();
for kernel_recvmsg() it does *not* - a couple of callers are using
it with non-NULL ->msg_control, which would be treated as userland
address on recvmsg side of things.

In all cases we are really setting a kvec-backed iov_iter, though.

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