fs: pstore: Replaced calls to kmalloc and memcpy with kmemdup

Replaced calls to kmalloc and memcpy with a single call to kmemdup.
This patch was found using coccicheck.

Signed-off-by: Alexandru Gheorghiu <gheorghiuandru@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>