pstore: Actually give up during locking failure

Without a return after the pr_err(), dumps will collide when two threads
call pstore_dump() at the same time.

Signed-off-by: Liu Hailong <liuhailong5@huawei.com>
Signed-off-by: Li Pengcheng <lipengcheng8@huawei.com>
Signed-off-by: Li Zhong <lizhong11@hisilicon.com>
[kees: improved commit message]
Signed-off-by: Kees Cook <keescook@chromium.org>