libext2fs: teach e2image imager to handle bigalloc file systems

Previously, "mke2fs -t ext4 -O bigalloc /tmp/foo.img 4G ;
e2image /tmp/foo.img /tmp/foo.e2i" would result in e2image
dumping core.  Fix ext2fs_image_bitmap_write() so it handles
file systems with bigalloc correctly.

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