UBI: add max_beb_per1024 to attach ioctl

This patch provides a possibility to set the "maximum expected number of
bad blocks per 1024 blocks" (max_beb_per1024) for each mtd device using
the UBI_IOCATT ioctl.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>