resize2fs: Allow (non-optimal) on-line resizing for flex_bg filesystems

resize2fs: Allow (non-optimal) on-line resizing for flex_bg filesystems

The current method of adding one block group at a time to a mounted
filesystem means it is impossible to accomodate the flex_bg allocation
method of placing the metadata together in a single block group.  For
now we "fix" this issue by using the traditional layout for new block
groups, where each block group is self-contained and contains its own
bitmap blocks and inode tables.  This means we don't get the layout
advantages of flex_bg in the new block groups, but at least it allows
on-line resizing to function.

Long term, we will need to create a new ioctl which does much more of
the resizing work in the kernel.

We also fix a bug in the ext3/ext4 ioctl fallback code so we stop
trying the ext3 ioctl for every single block group when resizing an
ext4 filesystem.

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