osd: use lower_bound to find head operation in pg_cancel_write_queue

std::multimap::find() is not guaranteed by the C++ standard to return the
first element of a range with equivalent keys, which may fail to retrieve
the head operation in pg.write_queue.

Switch to lower_bound() to reliably locate the head of the queue in O(log N)
time, and add an explicit check for it->first == oid to prevent falsely
matching a subsequent larger key when the target oid is missing.

src/osd/osd_primary_subops.cpp

@@ -875,8 +875,8 @@ void osd_t::submit_primary_rollback_subops(osd_op_t *cur_op, const osd_num_t* os
 
 void osd_t::pg_cancel_write_queue(pg_t & pg, osd_op_t *first_op, object_id oid, int retval)
 {
-    auto st_it = pg.write_queue.find(oid), it = st_it;
-    if (it == pg.write_queue.end() || it->second != first_op)
+    auto st_it = pg.write_queue.lower_bound(oid), it = st_it;
+    if (it == pg.write_queue.end() || it->first != oid || it->second != first_op)
     {
         // Write queue doesn't match the first operation.
         // first_op is a leftover operation from the previous peering of the same PG.