commit 816b0acf upstream.

If first_bad == this_sector when we get the WriteMostly disk
in read_balance(), valid disk will be returned with zero
max_sectors. It'll lead to a dead loop in make_request(), and
OOM will happen because of endless allocation of struct bio.

Since we can't get data from this disk in this case, so
continue for another disk.
Signed-off-by: Wei Fang <fangwei1@huawei.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>