switch vmsplice_to_user() to copy_page_to_iter()

I've switched the sanity checks on iovec to rw_copy_check_uvector();
we might need to do a local analog, if any behaviour differences are
not actually bugfixes here...

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

diff --cc fs/splice.c
Simple merge