fastboot: handle small flash block sizes correctly

Erase block sizes smaller than the ext4 block size may be valid, but
can incorrectly result in a stripe width smaller than the stride
size. Instead of reporting these sizes as invalid, add a check to
enforce that raid_stripe_width >= raid_stride.

Bug: 68770797
Test: Hack fb_getvar to report small erase block size, run fastboot
-w and confirm it does not print a warning or set stripe_width smaller
than stride.
Signed-off-by: Connor O'Brien <connoro@google.com>

Change-Id: I689ce4bdd5b38bd0952bb6de54785cca39176010