bluetooth: sdpd: switch to getpeereid() instead of LOCAL_PEERCRED

sdpd's usage was mostly fine, though it debatably should have been
checking the xucred version to avoid mishaps. Switch it to getpeereid()
anyways to avoid embedding xucred layout details in more program.

Reviewed by: emax
Differential Revision: https://reviews.freebsd.org/D51152