resize2fs, libext2fs: fix bugs in sparse_super2 support

resize2fs, libext2fs: fix bugs in sparse_super2 support

Fix resize2fs so that the location of the backup superblocks when the
sparse_super2 feature is enabled is properly set when growing the file
system from a single block group to larger file system sizes.

Also fix a bug where the block group summary statistics in some cases
when exapnding a sparse_super2 file system.

Finally, accurately calculate the file system metadata overhead of the
last block group in sparse_super2 file systems.

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