jffs2: fix hangs/crashs when not using CONFIG_JFFS2_PART_SIZE

Commit b5b004ad caused the sector_size to
be calculated incorrectly when the part size was not hardcoded.  This is
because the new code relied on part->size but tried to do the calculation
before it was initialized properly, and it did not take into consideration
the magic SIZE_REMAINING define.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>