crypto: api - Use work queue in crypto_destroy_instance

The function crypto_drop_spawn expects to be called in process
context.  However, when an instance is unregistered while it still
has active users, the last user may cause the instance to be freed
in atomic context.

Fix this by delaying the freeing to a work queue.

Fixes: 6bfd4809 ("[CRYPTO] api: Added spawns")
Reported-by: Florent Revest <revest@chromium.org>
Reported-by:  <syzbot+d769eed29cc42d75e2a3@syzkaller.appspotmail.com>
Reported-by:  <syzbot+610ec0671f51e838436e@syzkaller.appspotmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Florent Revest <revest@chromium.org>
Acked-by: Florent Revest <revest@chromium.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>