UBIFS: fix clean znode counter corruption in error cases

UBIFS maintains per-filesystem and global clean znode counters
('c->clean_zn_cnt' and 'ubifs_clean_zn_cnt'). It is important to maintain
correct values there since the shrinker relies on 'ubifs_clean_zn_cnt'.

However, in case of failures during commit the counters were corrupted. E.g.,
if a failure happens in the middle of 'write_index()', then some nodes in the
commit list ('c->cnext') are marked as clean, and some are marked as dirty. And
the 'ubifs_destroy_tnc_subtree()' frees does not retrun correct count, and we
end up with non-zero 'c->clean_zn_cnt' when unmounting. This means that if we
have 2 file-sytem and one of them fails, and we unmount it,
'ubifs_clean_zn_cnt' stays incorrect and confuses the shrinker.

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