datapath: Adopt Generic Netlink-compatible locking.

datapath: Adopt Generic Netlink-compatible locking.

The kernel Generic Netlink layer always holds a mutex (genl_lock) when it
invokes callbacks, so that means that there is no point in having
per-datapath mutexes or a separate vport lock.  This commit removes them.

This commit breaks support for Linux before 2.6.35 because it calls
genl_lock(), which wasn't exported before that version.  That problem will
be fixed once the whole userspace interface transitions to Generic
Netlink a few commits from now.

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