UBIFS: remove unnecessary stack variable

This is patch removes an unnecessary 'offs' variable from 'ubifs_wbuf_write_nolock()'
- we can just keep 'wbuf->offs' up-to-date instead. This patch is very minor
the only motivation for it was that it is cleaner to keep wbuf->offs up-to-date
by the time we call 'ubifs_leb_write()'.

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