dax: guarantee page aligned results from bdev_direct_access()

If a ->direct_access() implementation ever returns a map count less than
PAGE_SIZE, catch the error in bdev_direct_access().  This simplifies
error checking in upper layers.
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Reported-by: NRoss Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/block_dev.c

@@ -494,6 +494,8 @@ long bdev_direct_access(struct block_device *bdev, sector_t sector,
 	avail = ops->direct_access(bdev, sector, addr, pfn);
 	if (!avail)
 		return -ERANGE;
+	if (avail > 0 && avail & ~PAGE_MASK)
+		return -ENXIO;
 	return min(avail, size);
 }
 EXPORT_SYMBOL_GPL(bdev_direct_access);

fs/dax.c

@@ -52,7 +52,6 @@ int dax_clear_blocks(struct inode *inode, sector_t block, long size)
 		sz = min_t(long, count, SZ_128K);
 		clear_pmem(addr, sz);
 		size -= sz;
-		BUG_ON(sz & 511);
 		sector += sz / 512;
 		cond_resched();
 	} while (size);