Properly lock dp_mutex around changes to the datapath.

Properly lock dp_mutex around changes to the datapath.

We weren't locking dp_mutex() here but it really is necessary.  See the
comment on dp_mutex itself for details.

This actually restores some of the locking removed by commit 47b8652d
"Simplify use of dp_mutex."  That commit is correct that we can take
dp_mutex at a high level in dp_genl_openflow(), but it removes locking
from functions that are not called through dp_genl_openflow(): in
particular any Netlink command other than DP_GENL_C_OPENFLOW does not
go through that function, so those commands need to acquire the mutex
themselves.