resize2fs: fix r_bigalloc_big_expand test failure

resize2fs: fix r_bigalloc_big_expand test failure

Manually count the number free clusters in the last block group since
it might not be a multiple of 8, and using ext2fs_bitcount() might not
work if bitmap isn't properly padding out.

In addition, when setting up the block bitmap for the resized file
system, resize2fs was setting up the "real end" of the bitmap in units
of blocks instead of clusters.

We didn't notice this problem earlier because of a test failure which
caused the test to be skipped.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>