datapath: Hold mutex for DP while userspace is blocking on read.

datapath: Hold mutex for DP while userspace is blocking on read.

Currently we get a pointer to the DP in openvswitch_read() and
openvswitch_poll() and use it without any synchronization.  This means
that the DP could disappear from underneath us while we are using it.
Currently, this isn't a problem because userspace is single threaded but
it's better for the locking to be correct.

With this change we hold the mutex while doing a blocking wait, which
means that no changes can be made, including adding/removing flows.  It's
possible to make this finer grained but for the time being that isn't done,
since current userspace doesn't care.

Found with lockdep.

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