pstore/blk: Move verify_size() macro out of function

There's no good reason for the verify_size macro to live inside the
function. Move it up with the check_size() macro and fix indenting.

Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>