dm-snapshot: fix 'scheduling while atomic' on real-time kernels

dm-snapshot: fix 'scheduling while atomic' on real-time kernels

[ Upstream commit 8581b19eb2c5ccf06c195d3b5468c3c9d17a5020 ]

There is reported 'scheduling while atomic' bug when using dm-snapshot on
real-time kernels. The reason for the bug is that the hlist_bl code does
preempt_disable() when taking the lock and the kernel attempts to take
other spinlocks while holding the hlist_bl lock.

Fix this by converting a hlist_bl spinlock into a regular spinlock.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reported-by: Jiping Ma <jiping.ma2@windriver.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>