UBIFS: Add an assertion for clean_zn_cnt

This patch adds a new ubifs_assert() in ubifs_tnc_close() to check
if there are any leaks of per-filesystem @clean_zn_cnt. This new
assert inspects whether the return value of ubifs_destroy_tnc_subtree()
is equal to @clean_zn_cnt or not while umount.

Artem: a minor amendment

Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>