filesystem-dax: fix broken __dax_zero_page_range() conversion

The conversion of __dax_zero_page_range() to 'struct dax_operations'
caused it to frequently fail. The mistake was treating the @size
parameter as a dax mapping length rather than just a length of the
clear_pmem() operation. The dax mapping length is assumed to be hard
coded as PAGE_SIZE.

Without this fix any page unaligned zeroing request will trigger a
-EINVAL return from bdev_dax_pgoff().

Cc: Jan Kara <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Reported-by: Ross Zwisler <[e-mail]>
Tested-by: Ross Zwisler <[e-mail]>
Fixes: cccbce671582 ("filesystem-dax: convert to dax_direct_access()")
Signed-off-by: Dan Williams <[e-mail]>

fs/dax.c

···
 		void *kaddr;
 		pfn_t pfn;
 
-		rc = bdev_dax_pgoff(bdev, sector, size, &pgoff);
+		rc = bdev_dax_pgoff(bdev, sector, PAGE_SIZE, &pgoff);
 		if (rc)
 			return rc;
 
 		id = dax_read_lock();
-		rc = dax_direct_access(dax_dev, pgoff, PHYS_PFN(size), &kaddr,
+		rc = dax_direct_access(dax_dev, pgoff, 1, &kaddr,
 				&pfn);
 		if (rc < 0) {
 			dax_read_unlock(id);