UBIFS: fix budgeting calculations

The 'ubifs_release_dirty_inode_budget()' was buggy and incorrectly
freed the budget, which led to not freeing all dirty data budget.
This patch fixes that.

Also, this patch fixes ubifs_mkdir() which passed 1 in dirty_ino_d,
which makes no sense. Well, it is harmless though.

Also, add few more useful assertions. And improve few debugging
messages.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>