BUG/MEDIUM: quic: fix possible exit from qc_check_dcid() without unlocking

BUG/MEDIUM: quic: fix possible exit from qc_check_dcid() without unlocking

Locking of the CID tree was extended in qc_check_dcid() by recent commit
05f59a5 ("BUG/MINOR: quic: fix race condition in qc_check_dcid()") but
there was a direct return from the middle of the function which was not
covered by the unlock, resulting in the function keeping the lock on
success return.

Let's just remove this return and replace it with a variable to merge all
exit paths.

This must be backported wherever the fix above is backported.