portal_mtd: set writebufsize so UBI can attach to hyperfile MTD

[lacraig2]

Problem

handle_op_mtd_create() in src/portal/portal_mtd.c sets mtd->size,
mtd->erasesize, mtd->writesize, and mtd->oobsize, but leaves
mtd->writebufsize at its zero-initialized value.

UBI validates the write-buffer size when attaching a device
(ubi_attach_mtd_dev() in drivers/mtd/ubi/build.c): max_write_size must be
>= min_io_size (writesize), a multiple of it, and a power of 2. With
writebufsize == 0, attach fails immediately:

ubi0 error: ubi_attach_mtd_dev: bad write buffer size 0 for 2048 min. I/O unit

So ubiattach + mount -t ubifs cannot succeed on any portal/hyperfile-backed
MTD device, even with correct geometry.

Fix

Set mtd->writebufsize = req_writesize; (the page/write size), which satisfies
UBI's requirement. This lets UBI and UBIFS attach to hyperfile-backed MTD
partitions — needed to model firmware that stores nvram/config on a UBI-mounted
MTD partition.

Testing

Validated end-to-end against a kernel built with CONFIG_MTD_UBI +
CONFIG_UBIFS_FS: a NAND MTD device (128 KiB PEB / 2 KiB page) backed by a
mkfs.ubifs+ubinize image. Before: attach failed with the "bad write buffer
size 0" error above. After: ubiattach succeeds, UBI scans and attaches
(attached mtd0 ... size 16 MiB), and the guest mounts the UBIFS volume
(UBIFS: mounted UBI device 0, volume 0), reading its contents through the
genuine UBIFS → UBI → MTD path.