UBIFS: introduce helper functions for debugging checks and tests

This patch introduces helper functions for all debugging checks, so instead of
doing

if (!(ubifs_chk_flags & UBIFS_CHK_GEN))

we now do

if (!dbg_is_chk_gen(c))

This is a preparation to further changes where the flags will go away, and
we'll need to only change the helper functions, but the code which utilizes
them won't be touched.

At the same time this patch removes 'dbg_force_in_the_gaps()',
'dbg_force_in_the_gaps_enabled()', and dbg_failure_mode helpers for
consistency.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/log.c b/fs/ubifs/log.c
index affea94..fabfb53 100644
--- a/fs/ubifs/log.c
+++ b/fs/ubifs/log.c
@@ -752,7 +752,7 @@
 	struct ubifs_bud *bud;
 	long long bud_bytes = 0;
 
-	if (!(ubifs_chk_flags & UBIFS_CHK_GEN))
+	if (!dbg_is_chk_gen(c))
 		return 0;
 
 	spin_lock(&c->buds_lock);