sctp: fix warning at inet_sock_destruct() while release sctp socket

Commit 'net: Move rx skb_orphan call to where needed' broken sctp protocol
with warning at inet_sock_destruct(). Actually, sctp can do this right with
sctp_sock_rfree_frag() and sctp_skb_set_owner_r_frag() pair.

    sctp_sock_rfree_frag(skb);
    sctp_skb_set_owner_r_frag(skb, newsk);

This patch not revert the commit d55d87fd,
instead remove the sctp_sock_rfree_frag() function.

------------[ cut here ]------------
WARNING: at net/ipv4/af_inet.c:151 inet_sock_destruct+0xe0/0x142()
Modules linked in: sctp ipv6 dm_mirror dm_region_hash dm_log dm_multipath
scsi_mod ext3 jbd uhci_hcd ohci_hcd ehci_hcd [last unloaded: scsi_wait_scan]
Pid: 1808, comm: sctp_test Not tainted 2.6.31-rc2 #40
Call Trace:
 [<c042dd06>] warn_slowpath_common+0x6a/0x81
 [<c064a39a>] ? inet_sock_destruct+0xe0/0x142
 [<c042dd2f>] warn_slowpath_null+0x12/0x15
 [<c064a39a>] inet_sock_destruct+0xe0/0x142
 [<c05fde44>] __sk_free+0x19/0xcc
 [<...