libext2fs: add context to get_alloc_block

This patch add some contexts to the allocator about the block that is about
to be allocated.

The custom Android block allocator need a way to differentiate data block
and metadata block.

Test: cd external/e2fsprogs/lib && mma

Change-Id: I2899936a3a0043d26e062bf1e542483e9a6ac98f
3 files changed