resize2fs: create optimized flex_bg block groups

resize2fs: create optimized flex_bg block groups

Now that we are reserving all of the bg-specific metadata before we
try to allocate the metadata for the new block groups, we don't have
to temporarily disable the flex_bg feature flag while we allocate the
new metadata blocks --- this allows the newly created block groups to
have a much more optimized layout, instead of fragmenting the inode
table and block/inode bitmaps in sepraate block groups.

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