net/xfrm: Use an IS_ERR test rather than a NULL test

In case of error, the function xfrm_bundle_create returns an ERR
pointer, but never returns a NULL pointer. So a NULL test that comes
after an IS_ERR test should be deleted.

The semantic match that finds this problem is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@match_bad_null_test@
expression x, E;
statement S1,S2;
@@
x =  xfrm_bundle_create(...)
... when != x = E
*  if (x != NULL) 
S1 else S2
// </smpl>

Signed-off-by: Julien Brunel <brunel@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>