libext2fs: optimize ext2fs_allocate_group_table()

libext2fs: optimize ext2fs_allocate_group_table()

By using ext2fs_mark_block_bitmap_range2 and/or
ext2fs_block_alloc_stats_range(), we can significantly speed up the
time needed by mke2fs to allocate the inode table.

For example, the CPU time needed to run the command "mke2fs -t ext4
/tmp/foo.img 32T" (where tmpfs was mounted on /tmp) was decreased from
21.7 CPU seconds down to under 1.7 seconds.

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