pstore/blk: Introduce "best_effort" mode

In order to use arbitrary block devices as a pstore backend, provide a
new module param named "best_effort", which will allow using any block
device, even if it has not provided a panic_write callback.

Link: https://lore.kernel.org/lkml/20200511233229.27745-12-keescook@chromium.org/


Signed-off-by: Kees Cook <keescook@chromium.org>