namei: make may_follow_link() safe in RCU mode

We *can't* call that audit garbage in RCU mode - it's doing a weird
mix of allocations (GFP_NOFS, immediately followed by GFP_KERNEL)
and I'm not touching that... thing again.

So if this security sclero^Whardening feature gets triggered when
we are in RCU mode, tough - we'll fail with -ECHILD and have
everything restarted in non-RCU mode.  Only to hit the same test
and fail, this time with EACCES and with (oh, rapture) an audit spew
produced.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>