f2fs: add BUG cases to initiate fsck.f2fs

This patch replaces BUG cases with f2fs_bug_on to remain fsck.f2fs information.
And it implements some void functions to initiate fsck.f2fs too.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index abfbbca..46586d3 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -526,10 +526,10 @@
 	/* Update valid block bitmap */
 	if (del > 0) {
 		if (f2fs_set_bit(offset, se->cur_valid_map))
-			BUG();
+			f2fs_bug_on(sbi, 1);
 	} else {
 		if (!f2fs_clear_bit(offset, se->cur_valid_map))
-			BUG();
+			f2fs_bug_on(sbi, 1);
 	}
 	if (!f2fs_test_bit(offset, se->ckpt_valid_map))
 		se->ckpt_valid_blocks += del;