dm-raid: fix possible NULL dereference with undefined raid type

dm-raid: fix possible NULL dereference with undefined raid type

[ Upstream commit 2f6cfd6d7cb165a7af8877b838a9f6aab4159324 ]

rs->raid_type is assigned from get_raid_type_by_ll(), which may return
NULL. This NULL value could be dereferenced later in the condition
'if (!(rs_is_raid10(rs) && rt_is_raid0(rs->raid_type)))'.

Add a fail-fast check to return early with an error if raid_type is NULL,
similar to other uses of this function.

Found by Linux Verification Center (linuxtesting.org) with Svace.

Fixes: 33e53f06850f ("dm raid: introduce extended superblock and new raid types to support takeover/reshaping")
Signed-off-by: Alexey Simakov <bigalex934@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>