[JFFS2] Write buffer offset adjustment for NOR-ECC (Sibley) flash

After choosing new c->nextblock, don't leave the wbuf offset field
occasionally pointing at the start of the next physical eraseblock.
This was causing a BUG() on NOR-ECC (Sibley) flash, where we start
writing after the cleanmarker.

Among other this fix should cover write buffer offset adjustment
after flushing the last page of an eraseblock.

Signed-off-by: Alexander Belyakov <abelyako@googlemail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>