md/raid5: don't let shrink_slab shrink too far.

md/raid5: don't let shrink_slab shrink too far.

commit 49895bcc7e566ba455eb2996607d6fbd3447ce16 upstream.

I have a report of drop_one_stripe() called from
raid5_cache_scan() apparently finding ->max_nr_stripes == 0.

This should not be allowed.

So add a test to keep max_nr_stripes above min_nr_stripes.

Also use a 'mask' rather than a 'mod' in drop_one_stripe
to ensure 'hash' is valid even if max_nr_stripes does reach zero.

Fixes: edbe83ab4c27 ("md/raid5: allow the stripe_cache to grow and shrink.")
Reported-by: Tomas Papan <tomas.papan@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>