pstore/ram: Write new dumps to start of recycled zones

[ Upstream commit 9e5f1c19 ]

The ram_core.c routines treat przs as circular buffers. When writing a
new crash dump, the old buffer needs to be cleared so that the new dump
doesn't end up in the wrong place (i.e. at the end).

The solution to this problem is to reset the circular buffer state before
writing a new Oops dump.

Signed-off-by: Aleksandr Yashkin <a.yashkin@inango-systems.com>
Signed-off-by: Nikolay Merinov <n.merinov@inango-systems.com>
Signed-off-by: Ariel Gilman <a.gilman@inango-systems.com>
Link: https://lore.kernel.org/r/20191223133816.28155-1-n.merinov@inango-systems.com
Fixes: 896fc1f0 ("pstore/ram: Switch to persistent_ram routines")
[kees: backport to v4.9]
Link: https://lore.kernel.org/stable/157831399811194@kroah.com


Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>