crypto: null - Use spin lock instead of mutex

As the null algorithm may be freed in softirq context through
af_alg, use spin locks instead of mutexes to protect the default
null algorithm.

Reported-by:  <syzbot+b3e02953598f447d4d2a@syzkaller.appspotmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>