pstore/ram: Add check for kstrdup

Add check for the return value of kstrdup() and return the error
if it fails in order to avoid NULL pointer dereference.

Fixes: e163fdb3

 ("pstore/ram: Regularize prz label allocation lifetime")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230614093733.36048-1-jiasheng@iscas.ac.cn