UBIFS: do not allocate unneeded scan buffer

In 'ubifs_replay_journal()' we allocate 'sbuf' for scanning the log.
However, we already have 'c->sbuf' for these purposes, so do not
allocate yet another one. This reduces UBIFS memory consumption while
recovering.

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