af_key: Do not call xfrm_probe_algs in parallel

When namespace support was added to xfrm/afkey, it caused the
previously single-threaded call to xfrm_probe_algs to become
multi-threaded.  This is buggy and needs to be fixed with a mutex.

Reported-by: Abhishek Shah <abhishek.shah@columbia.edu>
Fixes: 283bc9f3 ("xfrm: Namespacify xfrm state/policy locks")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>