Specify arity of "dcp/backfill ScanDefer" phosphor trace point
Specify arity of "dcp/backfill ScanDefer" phosphor trace point

The arity of the tracepoint was accidentally missed out when
it was added which means the argument name "seqno" was being stored
as an argument rather than as an argument name.

Change-Id: I09e1f73773063428034d69724d18b971cd2a871c
Reviewed-on: http://review.couchbase.org/97833
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Tim Bradgate <f1a26ee65f021cb7d82b83dfb09abf144d3eb64f@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine