iov_iter_get_pages(): sanity-check arguments

zero maxpages is bogus, but best treated as "just return 0";
NULL pages, OTOH, should be treated as a hard bug.

get rid of now completely useless checks in xarray_get_pages{,_alloc}().

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>