resize2fs: Fix support for filesystems with the uninit_bg feature

resize2fs: Fix support for filesystems with the uninit_bg feature

If the filesystem has the uninit_bg feature, then parts of the block
and inode bitmap may not be initialized.  Teach resize2fs how to deal
with these case appropriately.  (Most of these fixes were fortunately
not necessary for the common case where the resize_inode is present to
reserve space, and where the filesystem is being expanded instead of
being shrunk.)

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