flat_mutation_reader: Fix use-after-scope on timeout
flat_mutation_reader: Fix use-after-scope on timeout

timeout parameter was captured by reference, and could be accessed out
of scope in case the repeat loop deferred.

Fixes debug-mode failure of flat_mutation_reader_test.

Message-Id: <99ee7a8cd0ef1d0fda05147114ea44c858a20c25@scylladb.com>

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