libext2fs: skip start_blk adjustment when stride and flex_bg is set

libext2fs: skip start_blk adjustment when stride and flex_bg is set

Currently some stride optimization is done in
ext2fs_allocate_group_table() by adjusting start_blk block where we
start allocating block, or inode bitmaps.

However in flex_bg case this is currently useless since the values are
going to be overridden anyway. Moreover in flex_bg case the group might
already be full and the stride optimization will fail. As a result file
system resize might fail needlessly in some situations.

It can be shown by this example:

mke2fs -b 1024 -i 1024 -E stride=8192 -t ext4 /dev/loop0 1024000
resize2fs /dev/loop0 102400000
resize2fs 1.43.5 (04-Aug-2017)
Resizing the filesystem on /dev/loop0 to 102400000 (1k) blocks.
./resize/resize2fs: Could not allocate block in ext2 filesystem while trying to resize /dev/loop0
Please run 'e2fsck -fy /dev/loop0' to fix the filesystem
after the aborted resize operation.

Fix this by not doing the stride adjustment in case of flex_bg.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>