mtd: block2mtd: initialize writebufsize

The writebufsize concept was introduce by commit
"0e4ca7e5 mtd: add writebufsize field to mtd_info struct" and it represents
the maximum amount of data the device writes to the media at a time. This is
an important parameter for UBIFS which is used during recovery and which
basically defines how big a corruption caused by a power cut can be.

However, we forgot to set this parameter for block2mtd. Set it to PAGE_SIZE
because this is actually the amount of data we write at a time.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Acked-by: Joern Engel <joern@lazybastard.org>
Cc: stable@kernel.org [2.6.38+]
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>