reisze2fs: sanity check free block group counts when calculating minimum size

reisze2fs: sanity check free block group counts when calculating minimum size

If one or more block group descriptor's free blocks count is insane,
it's possible this can lead to a infinite loop in the function
calculate_minimum_resize_size(), which is called by resize2fs -P or
resize2fs -M.

Add some sanity checks to avoid this.  In the case where the file
system is corrupt, this will result in resize2fs -P reporting an
incorrect value, but that's OK, since when we try to do an actual
resize operation, resize2fs requires that the file system be freshly
checked using e2fsck.

https://github.com/tytso/e2fsprogs/issues/94

Fixes: ac94445fc01f ("resize2fs: make minimum size estimates more reliable for mounted fs")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>