Bluetooth: host: l2cap deadlock fix

[jori-nordic]

This combination of commits fixes the symptoms seen in #34600. It also adds an multilink l2cap test to stress the host in case other bugs like this crop up.

Fixes #34600.

[jori-nordic]

~~whoops looks like I forgot to review the test, it's still littered with commented code and is missing the new alloc_seg callback. Adding DNM until I fix it. The rest of the PR can still be reviewed.~~

Whole PR is now ready for review.

[carlescufi]

@Vudentz a review from you would be appreciated for this PR

[jori-nordic]

Sorry for the force-pushing @jhedberg I just noticed the checkpatch warnings.

[jhedberg]

@jori-nordic no worries, I'll approve again once everything is green. There's still a failure due to missing commit message body in one of the commits.