resize2fs: reserve all metadata blocks for flex_bg file systems

resize2fs: reserve all metadata blocks for flex_bg file systems

For flex_bg file systems, if we need to relocate an allocation bitmap
or inode table, we need to make sure that all metadata blocks have
been reserved, lest we end up overwriting a metadata block belonging
to a different block group.

This change fixes the following test case:

rm -f foo.img; touch foo.img
truncate -s 32G foo.img
mke2fs -F -t ext4 -E resize=12582912 foo.img
e2fsck -f foo.img
truncate -s 64G foo.img
./resize2fs foo.img
e2fsck -fy foo.img

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