Most of the users immediately follow successful iov_iter_get_pages()
with advancing by the amount it had returned.

Provide inline wrappers doing that, convert trivial open-coded
uses of those.

BTW, iov_iter_get_pages() never returns more than it had been asked
to; such checks in cifs ought to be removed someday...
Reviewed-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>