Retry ioctl in uncrypt if it returns block# 0

In some conditions, ioctl(fd, FIBMAP, &block) returns block number 0.This
is a failure to locate the actual block number of the update package and
will result in an invalid block.map. This CL retries ioctl a few times
if it returns block number as 0.

Bug: 31632090
Test: On N9, uncrypt retries ioctl and produces the correct blockmap.
Change-Id: I913f98cf5c112915c2e803d0683db273c89053b6