Rather than adjusting max_nr_stripes whenever {grow,drop}_one_stripe()
succeeds, do it inside the functions.

Also choose the correct hash to handle next inside the functions.

This removes duplication and will help with future new uses of
{grow,drop}_one_stripe.

This also fixes a minor bug where the "md/raid:%md: allocate XXkB"
message always said "0kB".
Signed-off-by: NeilBrown <neilb@suse.de>