UBIFS: harmonize znode flag helpers

We have 3 znode flags: cow, obsolete, dirty. For the last flag we have a
'ubifs_zn_dirty()' helper function, but for the other 2 flags we use
'test_bit()' directly.

This patch makes the situation more consistent and introduces helpers for the
other 2 flags: 'ubifs_zn_cow()' and 'ubifs_zn_obsolete()'.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/commit.c b/fs/ubifs/commit.c
index 8ab03d1..637e076 100644
--- a/fs/ubifs/commit.c
+++ b/fs/ubifs/commit.c
@@ -78,7 +78,7 @@
 	 * If the root TNC node is dirty, we definitely have something to
 	 * commit.
 	 */
-	if (c->zroot.znode && test_bit(DIRTY_ZNODE, &c->zroot.znode->flags))
+	if (c->zroot.znode && ubifs_zn_dirty(c->zroot.znode))
 		return 0;
 
 	/*