Followup fixes on the mtdparts spread patchset

Consolidate some code in mtd_get_len_incl_bad(), and fix a condition
where a valid partition could be reported as truncated if it has a
good block at the end of the device (unlikely, since the BBT is usually
there).

Fix mid-block declarations in net_part_size().

Signed-off-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Ben Gardiner <bengardiner@nanometrics.ca>