iomap: turn the byte variable in iomap_zero_iter into a ssize_t

@bytes also holds the return value from iomap_write_end, which can
contain a negative error value.  As @bytes is always less than the page
size even the signed type can hold the entire possible range.

Fixes: c6f4046 ("fsdax: decouple zeroing from the iomap buffered I/O code")
Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dan Williams <[email protected]>

Author: Christoph Hellwig

fs/iomap/buffered-io.c

@@ -883,7 +883,7 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
 
 	do {
 		unsigned offset = offset_in_page(pos);
-		size_t bytes = min_t(u64, PAGE_SIZE - offset, length);
+		ssize_t bytes = min_t(u64, PAGE_SIZE - offset, length);
 		struct page *page;
 		int status;