[XFS] Don't use kmap in xfs_iozero.

kmap() is inefficient and does not scale well. kmap_atomic() is a better
choice. Use the generic wrapper function instead of open coding the
kmap-memset-dcache flush-kunmap stuff.

SGI-PV: 960904
SGI-Modid: xfs-linux-melb:xfs-kern:28041a
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NChristoph Hellwig <hch@infradead.org>
Signed-off-by: NTim Shimmin <tes@sgi.com>

fs/xfs/linux-2.6/xfs_lrw.c

@@ -137,7 +137,6 @@ xfs_iozero(
 	unsigned		bytes;
 	struct page		*page;
 	struct address_space	*mapping;
-	char			*kaddr;
 	int			status;
 
 	mapping = ip->i_mapping;
@@ -155,15 +154,13 @@ xfs_iozero(
 		if (!page)
 			break;
 
-		kaddr = kmap(page);
 		status = mapping->a_ops->prepare_write(NULL, page, offset,
 							offset + bytes);
-		if (status) {
+		if (status)
 			goto unlock;
-		}
 
-		memset((void *) (kaddr + offset), 0, bytes);
-		flush_dcache_page(page);
+		memclear_highpage_flush(page, offset, bytes);
+
 		status = mapping->a_ops->commit_write(NULL, page, offset,
 							offset + bytes);
 		if (!status) {
@@ -172,7 +169,6 @@ xfs_iozero(
 		}
 
 unlock:
-		kunmap(page);
 		unlock_page(page);
 		page_cache_release(page);
 		if (status)