MB-33463: Set vbid once for compaction context

When the compaction callback is made, we only need to set the vbid once
rather than for each item. This reduces code path.

Change-Id: I2479521c36fee8c6e80b9a4669e1254c5788bde0
Reviewed-on: http://review.couchbase.org/112978
Reviewed-by: Dave Rigby <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: scottlashley

engines/ep/src/magma-kvstore/magma-kvstore.cc

@@ -377,9 +377,9 @@ bool MagmaKVStore::compactionCallBack(MagmaKVStore::MagmaCompactionCB& cbCtx,
         logger->TRACE("MagmaCompactionCB {}", itemString.str());
     }
 
-    cbCtx.vbid = getVbid(metaSlice);
-
     if (!cbCtx.initialized) {
+        cbCtx.vbid = getVbid(metaSlice);
+
         // If we have a compaction_ctx, that means we are doing an
         // explicit compaction triggered by the kv_engine.
         {