rxrpc: rxrpc_connection_lock shouldn't be a BH lock, but conn_lock is

rxrpc_connection_lock shouldn't be accessed as a BH-excluding lock.  It's
only accessed in a few places and none of those are in BH-context.

rxrpc_transport::conn_lock, however, *is* a BH-excluding lock and should be
accessed so consistently.

Signed-off-by: David Howells <dhowells@redhat.com>