kernel: don't switch allocate_partition to use mtd_roundup_to_eb

kernel: don't switch allocate_partition to use mtd_roundup_to_eb

This mtd_roundup_to_eb helper was introduced years ago in the commit
daec7ad768841 ("kernel/3.10: add separate rootfs partition parser") and
it was probably supposed to simplify code a bit.

With the recent upstream commit 1eeef2d7483a7 ("mtd: handle partitioning
on devices with 0 erasesize") the logic in allocate_partition got
slightly more complex and we can't use this simple helper anymore as it
doesn't support MTD_NO_ERASE properly.

There also isn't any real gain from this helper, so it's probably easier
to just don't use it *or* work on upstreaming it to avoid maintenance
cost.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>