mtd: add an mtd method for get_len_incl_bad()

The logic to 'spread' mtd partitions needs to calculate the length in
the mtd device, including bad blocks.

This patch introduces a new function, mtd_get_len_incl_bad that can
return both the length including bad blocks and whether that length
was truncated on the device. This new function will be used by the
mtdparts spread command later in this series. The definition of the
function is #ifdef'd out in configurations that do not use the new
'mtdparts spread' command.

Signed-off-by: Ben <Gardiner&lt;bengardiner@nanometrics.ca>
CC: Scott Wood <scottwood@freescale.com>