UBIFS: do not read flash unnecessarily

This fix makes the 'dbg_check_old_index()' function return
immediately if debugging is disabled, instead of executing
incorrect 'goto out' which causes UBIFS to:

1. Allocate memory
2. Read the flash

On every commit. OK, we do not commit that often, but it is
still silly to do unneeded I/O anyway.

Credits to coverity for spotting this silly issue.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: stable@kernel.org