UBIFS: share the next_log_lnum helper

We'll need to use the 'next_log_lnum()' helper function from log.c in the fixup
code, so let's move it to misc.h. IOW, this is a preparation to the following
free space fixup changes.

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