l2tp: cleanup comparisons to NULL

checkpatch warns about comparisons to NULL, e.g.

        CHECK: Comparison to NULL could be written "!rt"
        #474: FILE: net/l2tp/l2tp_ip.c:474:
        +       if (rt == NULL) {

These sort of comparisons are generally clearer and more readable
the way checkpatch suggests, so update l2tp accordingly.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>