mke2fs: fix bugs in hugefile creation

mke2fs: fix bugs in hugefile creation

For certain sizes mke2fs's hugefile creation would fail with the error:

mke2fs: Could not allocate block in ext2 filesystem while creating huge file 0

This would happen because we had failed to reserve enough space for
the metadata blocks for the hugefile.  There were two problems:

1) The overhead calculation function failed to take into account the
cluster size for bigalloc file systems.

2) In the case where num_blocks is 0 and num_files is 1, the overhead
calculation function was passed a size of 0, which caused the
calculated overhead to be zero.

Google-Bug-Id: 123239032
Signed-off-by: Theodore Ts'o <tytso@mit.edu>