tcp: Clear sack info on purge

This ensures that the sack information is cleared when TCP is in
FIN-WAIT1 state.

This also tracks whenever tcp queue purge is called which resets
all the transmit queue information and ensures there are no
outstanding packets.

This addresses the following -

 16496.596106:   <6> Unable to handle kernel paging request at virtual
		     address fffffff2cd81a368
 16496.730771:   <2> pc : tcp_ack+0x174/0x11e8
 16496.734536:   <2> lr : tcp_rcv_state_process+0x318/0x1300
 16497.183109:   <2> Call trace:
 16497.183114:   <2>  tcp_ack+0x174/0x11e8
 16497.183115:   <2>  tcp_rcv_state_process+0x318/0x1300
 16497.183117:   <2>  tcp_v4_do_rcv+0x1a8/0x1f0
 16497.183118:   <2>  tcp_v4_rcv+0xe90/0xec8
 16497.183120:   <2>  ip_protocol_deliver_rcu+0x150/0x298
 16497.183121:   <2>  ip_local_deliver+0x21c/0x2a8
 16497.183122:   <2>  ip_rcv+0x1c4/0x210
 16497.183124:   <2>  __netif_receive_skb_core+0xab0/0xd90
 16497.183125:   <2>  netif_receive_skb_internal+0x12c/0x368
 16497.183126:   <2>  napi_gro_receive+0x1e0/0x290

No outstanding data is expected on a socket after write queue purge
is called.

CRs-Fixed: 2541019
Change-Id: I586c43931969eae48fe1bf9799ef38267401fdc6
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>