xfrm: convert empty xfrm_audit_* macros to functions

it removes these warnings when CONFIG_AUDITSYSCALL is unset:

net/xfrm/xfrm_user.c: In function 'xfrm_add_sa':
net/xfrm/xfrm_user.c:412: warning: unused variable 'sid'
net/xfrm/xfrm_user.c:411: warning: unused variable 'sessionid'
net/xfrm/xfrm_user.c:410: warning: unused variable 'loginuid'
net/xfrm/xfrm_user.c: In function 'xfrm_del_sa':
net/xfrm/xfrm_user.c:485: warning: unused variable 'sid'
net/xfrm/xfrm_user.c:484: warning: unused variable 'sessionid'
net/xfrm/xfrm_user.c:483: warning: unused variable 'loginuid'
net/xfrm/xfrm_user.c: In function 'xfrm_add_policy':
net/xfrm/xfrm_user.c:1132: warning: unused variable 'sid'
net/xfrm/xfrm_user.c:1131: warning: unused variable 'sessionid'
net/xfrm/xfrm_user.c:1130: warning: unused variable 'loginuid'
net/xfrm/xfrm_user.c: In function 'xfrm_get_policy':
net/xfrm/xfrm_user.c:1382: warning: unused variable 'sid'
net/xfrm/xfrm_user.c:1381: warning: unused variable 'sessionid'
net/xfrm/xfrm_user.c:1380: warning: unused variable 'loginuid'
net/xfrm/xfrm_user.c: In function 'xfrm_add_pol_expire':
net/xfrm/xfrm_user.c:1620: warning: unused variable 'sid'
net/xfrm/xfrm_user.c:1619: warning: unused variable 'sessionid'
net/xfrm/xfrm_user.c:1618: warning: unused variable 'loginuid'
net/xfrm/xfrm_user.c: In function 'xfrm_add_sa_expire':
net/xfrm/xfrm_user.c:1658: warning: unused variable 'sid'
net/xfrm/xfrm_user.c:1657: warning: unused variable 'sessionid'
net/xfrm/xfrm_user.c:1656: warning: unused variable 'loginuid'

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>