Bug 5329: cbdata.cc:276 "c->locks > 0" assertion on reconfigure (#1625)

Bug 5329: cbdata.cc:276 "c->locks > 0" assertion on reconfigure (#1625)

Recent commit 0f78379 correctly removed an excessive cbdata lock of a
CachePeer::digest object in peerDigestCreate() but accidentally lost
another digest lock while inlining peerDigestCreate(). The resulting
excessive unlocking triggered reconfiguration assertions.

This change restores the lost lock as a short-term fix.

Long-term, CachePeer code should be fixed to become an exclusive[^1]
PeerDigest owner (i.e. creating and deleting its cbdata-protected digest
object without locking, unlocking, or checking locks). That improvement
is already in the works, but it requires significant code refactoring.

[^1]: Shared PeerDigest ownership (i.e. reference counting instead of
explicit delete and cbdata) does not work well in this context due to
circular references.