imgact_binmisc: minor re-organization of imgact_binmisc_exec exits

Notably, streamline error paths through the existing 'done' label, making it
easier to quickly verify correct cleanup.

Future work might add a kernel-only flag to indicate that a interpreter uses
#a. Currently, all executions via imgact_binmisc pay the penalty of
constructing sname/fname, even if they will not use it. qemu-user-static
doesn't need it, the stock rc script for qemu-user-static certainly doesn't
use it, and I suspect these are the vast majority of (if not the only)
current users.

MFC after: 1 week