lwt: fix possible leak of "prune" counter
lwt: fix possible leak of "prune" counter

If get_schema_for_read() fails "prune" counter will not be decremented.
The patch fixes it by creating RAI object earlier. Also return releasing
of a mutation in release_mutation() which was dropped by mistake.

Fixes #6124

Message-Id: <d43c9e334c41a674d31d738bf4ac3cb3f9567a83@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla