Fix verity on system partitions larger than 2G

If the system partition is larger than 2G, the device fails to read
verity metadata, because fseek accepts only a 32-bit signed offset.
Switch from fseek to lseek64 to allow seeking using a 64-bit offset,
which solves the problem. At the same time, move away from stdio in
the function.

Bug: 17705619
Change-Id: I226320498dcb750ec6cde84411c7fe0774c9cab7