datapath: Merge do_destroy_dp into destroy_dp.

datapath: Merge do_destroy_dp into destroy_dp.

Both do_destroy_dp() and destroy_dp() are small functions and
only have a single caller.  There's no good reason for them to
be separate so this merges them together.  It also makes things
more logically consistent and easier to read in the next commit,
which adds additional locking as everything is in one place.

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