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: c6f40468657d ("fsdax: decouple zeroing from the iomap buffered I/O code")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211208091203.2927754-1-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>