resize2fs: don't interpret bitmap shift while crossing flexbg as raid stride

resize2fs: don't interpret bitmap shift while crossing flexbg as raid stride

resize2fs tries to infer the RAID stride by observing differences
between the locations of adjacent block groups' block and inode
bitmaps within the block group.  If the two block groups being
compared belong to different flexbgs, however, it'll be fooled by the
large offset into thinking that the FS has an abnormally large RAID
stride.

Therefore, teach it not to get confused by crossing a flexbg.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reported-by: TR Reardon <thomas_reardon@hotmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>