[INET]: Fix inet_diag dead-lock regression

[INET]: Fix inet_diag dead-lock regression

The inet_diag register fix broke inet_diag module loading because the
loaded module had to take the same mutex that's already held by the
loader in order to register the new handler.

This patch fixes it by introducing a separate mutex to protect the
handling of handlers.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>