xfrm: update outdated comment

The function __xfrm4_bundle_create() was refactored into
xfrm_bundle_create() (among others) by commit 25ee3286dcbc
("xfrm: Merge common code into xfrm_bundle_create"). The
responsibility for setting dst->obsolete to DST_OBSOLETE_FORCE_CHK
now lives in xfrm_bundle_create(). Update the comment accordingly.

Signed-off-by: kexinsun <kexinsun@smail.nju.edu.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>