imgact_elf: Ensure that the return value in parse_notes is initialized

[markj]

Summary

parse_notes relies on the caller-supplied callback to initialize "res".
Two callbacks are used in practice, brandnote_cb and note_fctl_cb, and
the latter fails to initialize res. Try to fix it.

Reported by: KMSAN

[kib]

I.e. the only effect of this was that the inner loop from check_note potentially did not stopped after fctl note was handled, but continued till the end of the phdrs.

[markj]

In D29986#672701, @kib wrote:

I.e. the only effect of this was that the inner loop from check_note potentially did not stopped after fctl note was handled, but continued till the end of the phdrs.

I think so, yes. I will note this in the commit message.