UBIFS: Add missing break statements in dbg_chk_pnode()

This is a minor fix. These two branches in 'dbg_chk_pnode()'
are dealing with different conditions. Although there is
no fault in current state, I think adding "break"s in
each end of branch is better.

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