Correctly calculate tot_used_blocks on ext4 with uninit_bg

The calculated number of blocks to encrypt is too high on ext4
filesystems that have the uninit_bg feature.  This is because the
calculation assumes that all blocks not counted in bg_free_blocks_count
need to encrypted.  But actually, uninitialized block groups have inode
blocks which vold doesn't encrypt since they are uninitialized, but they
are "allocated" and thus reduce bg_free_blocks_count.

Therefore, add a helper function num_base_meta_blocks_in_group() which
returns the number of blocks to encrypt in an uninitialized block group.
Use it both for the encryption and for calculating 'tot_used_blocks'.

Also compute 'tot_used_blocks' additively rather than subtractively, as
this is easier to understand.

Test: see I08fc8465f7962abd698904b5466f3ed080d53953
Change-Id: I4d2cb40291da67dd1bafd61289ccb9e6343bfda3