sctp: move the active_key update after sh_keys is added

In commit 58acd100 ("sctp: update active_key for asoc when old key is
being replaced"), sctp_auth_asoc_init_active_key() is called to update
the active_key right after the old key is deleted and before the new key
is added, and it caused that the active_key could be found with the key_id.

In Ying Xu's testing, the BUG_ON in sctp_auth_asoc_init_active_key() was
triggered:

  [ ] kernel BUG at net/sctp/auth.c:416!
  [ ] RIP: 0010:sctp_auth_asoc_init_active_key.part.8+0xe7/0xf0 [sctp]
  [ ] Call Trace:
  [ ]  sctp_auth_set_key+0x16d/0x1b0 [sctp]
  [ ]  sctp_setsockopt.part.33+0x1ba9/0x2bd0 [sctp]
  [ ]  __sys_setsockopt+0xd6/0x1d0
  [ ]  __x64_sys_setsockopt+0x20/0x30
  [ ]  do_syscall_64+0x5b/0x1a0

So fix it by moving the active_key update after sh_keys is added.

Fixes: 58acd100 ("sctp: update active_key for asoc when old key is being replaced")
Reported-by: Ying Xu <yinxu@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off...