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>

lib/iov_iter.c

@@ -1368,9 +1368,6 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
 	size_t size = maxsize;
 	loff_t pos;
 
-	if (!size || !maxpages)
-		return 0;
-
 	pos = i->xarray_start + i->iov_offset;
 	index = pos >> PAGE_SHIFT;
 	offset = pos & ~PAGE_MASK;
@@ -1440,10 +1437,11 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 
 	if (maxsize > i->count)
 		maxsize = i->count;
-	if (!maxsize)
+	if (!maxsize || !maxpages)
 		return 0;
 	if (maxsize > MAX_RW_COUNT)
 		maxsize = MAX_RW_COUNT;
+	BUG_ON(!pages);
 
 	if (likely(user_backed_iter(i))) {
 		unsigned int gup_flags = 0;
@@ -1522,9 +1520,6 @@ static ssize_t iter_xarray_get_pages_alloc(struct iov_iter *i,
 	size_t size = maxsize;
 	loff_t pos;
 
-	if (!size)
-		return 0;
-
 	pos = i->xarray_start + i->iov_offset;
 	index = pos >> PAGE_SHIFT;
 	offset = pos & ~PAGE_MASK;