raid5: guarantee enough stripes to avoid reshape hang

If there aren't enough stripes, reshape will hang. We have a check for
this in new reshape, but miss it for reshape resume, hence we could see
hang in reshape resume. This patch forces enough stripes existed if
reshape resumes.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>