mtd: Collect bad block count for ecc stats lazily.

Normally bad block counts for ECC stats are collected during boot time.
This can be done lazily when the ECCGETSTATS ioctl is invoked on the
partition. This can significantly decrease boot time, depending on the
size of the partition. Also rescanning on every ioctl invocation helps
in having the latest bad block count rather than depending on the count
that is collected during boot.

Change-Id: I43d7a769a1d4ef769823d0b5bbe132adb474f892
Signed-off-by: Murali Palnati <palnatim@codeaurora.org>