fs_mgr: When formating ext4 volumes, respect the length parameter

When formatting a volume because it is marked as formattable, the
length parameter is discarded which makes fs_mgr write a filesystem
to the full length of the block device.

This patch adds length semantics to ext4 formatting, if the length
is greater than zero, use that, if it isn't subtract that size
from the block size.

Change-Id: I4d18d5161360f8de0e571352e705678682e61332
Ticket: CYNGNOS-931