resize2fs: fix off-line resize of file systems with flex_bg && !resize_inode

resize2fs: fix off-line resize of file systems with flex_bg && !resize_inode

When doing an off-line resize2fs of an initially very small file
system, it's possible to run out of reserved gdt blocks (which are
reserved via the resize inode).  Once we run out, we need to move the
allocation bitmaps and inode table out of the way to grow the gdt
blocks.  Unfortunately, when moving these metadata blocks, it was
possible that a block that had been just been newly allocated for a
new block group could also get allocated for a metadata block for an
existing block group that was being moved.

To prevent this, after we grow the gdt blocks and allocate the
metadata blocks for the new block groups, make sure all of these
blocks are marked as reserved.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: John Jolly <john.jolly@gmail.com>