pstore: Warn on PSTORE_TYPE_PMSG using deprecated function

PMSG now uses ramoops_pstore_write_buf_user() instead of ...write_buf().
Print a ratelimited warning if gets accidentally called.

Signed-off-by: Joel Fernandes <joelaf@google.com>
[kees: adjusted commit log and added -EINVAL return]
Signed-off-by: Kees Cook <keescook@chromium.org>