resize2fs: Fix data corruption bug when growing an ext4 filesystem off-line

resize2fs: Fix data corruption bug when growing an ext4 filesystem off-line

When allocating a new set of block group metadata as part of growing
the filesystem, the resize2fs code assumes that the bitmap and inode
table blocks are in their own block group; an assumption which is
changed by the flex_bg feature.  This commit works around the problem
by temporarily turning off flex_bg while allocating the new block
group metadata, to avoid potentially overwriting previously allocated
data blocks.

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