net: xfrm: Fix end of loop tests for list_for_each_entry

The list_for_each_entry() iterator, "pos" in this code, can never be
NULL so the warning will never be printed.

Signed-off-by: Harshvardhan Jha <harshvardhan.jha@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>