MdeModulePkg/Dhcp4Dxe: Free NET_BUF data after sent out to avoid memory leak

MdeModulePkg/Dhcp4Dxe: Free NET_BUF data after sent out to avoid memory leak

* When build a DHCP message in function DhcpSendMessage() or DhcpRetransmit(),
a new NET_BUF is created by the library of NetbufFromExt, but it's not freed
after it is sent out. This patch is to fix this memory leak issue.

V2:
* Since packet has already been referred by DhcpSb->LastPacket, and will be
freed when sending another packet or clean up, there is no need to add an
extra free function in NetbufFromExt.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wang Fan <fan.wang@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>