Core/Movement: Knockback Effect on Flying Mount

by @robinsch; closes #6099

Keep the mounted player flying after the knockback effect cast on mounted player.

Before these changes, the flying mount behaves like a ground mount after the knockback.
From large enough heights, the fall causes damage or death to the falling (mounted) character.

src/server/game/Entities/Unit/Unit.cpp

@@ -16448,7 +16448,7 @@ void Unit::KnockbackFrom(float x, float y, float speedXY, float speedZ)
         float vcos, vsin;
         GetSinCos(x, y, vsin, vcos);
 
-        WorldPacket data(SMSG_MOVE_KNOCK_BACK, (8+4+4+4+4+4));
+        WorldPacket data(SMSG_MOVE_KNOCK_BACK, (8 + 4 + 4 + 4 + 4 + 4));
         data << GetPackGUID();
         data << uint32(0);                                      // counter
         data << float(vcos);                                    // x direction
@@ -16457,6 +16457,9 @@ void Unit::KnockbackFrom(float x, float y, float speedXY, float speedZ)
         data << float(-speedZ);                                 // Z Movement speed (vertical)
 
         player->GetSession()->SendPacket(&data);
+        
+        if (player->HasAuraType(SPELL_AURA_MOD_INCREASE_MOUNTED_FLIGHT_SPEED) || player->HasAuraType(SPELL_AURA_FLY))
+            player->SetCanFly(true, true);
     }
 }
 
@@ -17515,7 +17518,7 @@ bool Unit::SetSwim(bool enable)
     return true;
 }
 
-bool Unit::SetCanFly(bool enable)
+bool Unit::SetCanFly(bool enable, bool /*packetOnly = false */)
 {
     if (enable == HasUnitMovementFlag(MOVEMENTFLAG_CAN_FLY))
         return false;