datapath: Acquire dp->mutex when deleting a datapath.

datapath: Acquire dp->mutex when deleting a datapath.

It's possible that someone is using the datapath data structures
when we attempt to delete the datapath.  The first writer will
only hold dp->mutex, which we don't currently acquire when deleting.
This adds that lock to prevent a potential race (this can't currently
happen because userspace is single threaded, as long as "ovs-dpctl
del-dp" is not used at the same time).

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>