ubi: Add scrub_all support to UBI

The data on NAND parts degrades overtime. This is known as data decay.
This decay is accelarated by extreme temperatures.

When data on nand is read over and over again this can also cause decay
of data. This is known as read disturb and can degrade the data in
the page/block that is read and the adjacent cells as well.

This data degrade can be corrected to a certain degree by the nand
driver/controller using the ECC but this is not sufficient specially
in products designed to last years.

The only way to combat the decay is to "refresh" the data by moving it
to a new location.

Add scrub_all support to UBI to facilitate the data refresh.

Change-Id: Ifafb82fe4ddb7120277dcfbbff79b3e087ca344d
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>