btrfs: Remove EXTENT_WRITEBACK

This flag was introduced in a52d9a8033c4 ("Btrfs: Extent based page
cache code.") and subsequently it's usage effectively was removed by
1edbb734b4e0 ("Btrfs: reduce CPU usage in the extent_state tree") and
f2a97a9dbd86 ("btrfs: remove all unused functions"). Just remove it,
no functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index ee246a7..5725adf 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4229,8 +4229,7 @@ int try_release_extent_mapping(struct page *page, gfp_t mask)
 			}
 			if (!test_range_bit(tree, em->start,
 					    extent_map_end(em) - 1,
-					    EXTENT_LOCKED | EXTENT_WRITEBACK,
-					    0, NULL)) {
+					    EXTENT_LOCKED, 0, NULL)) {
 				set_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
 					&btrfs_inode->runtime_flags);
 				remove_extent_mapping(map, em);