Btrfs: Make sure pages are dirty before doing delalloc for them

This adds a PageDirty check to the writeback path that locks pages
for delalloc.  If a page wasn't dirty at this point, it is in the
process of being truncated away.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index ad75a9c..69ea096 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1205,7 +1205,8 @@
 			 */
 			if (pages[i] != locked_page) {
 				lock_page(pages[i]);
-				if (pages[i]->mapping != inode->i_mapping) {
+				if (!PageDirty(pages[i]) ||
+				    pages[i]->mapping != inode->i_mapping) {
 					ret = -EAGAIN;
 					unlock_page(pages[i]);
 					page_cache_release(pages[i]);