datapath: Drop constness of datapath pointers.

datapath: Drop constness of datapath pointers.

A few places marked struct datapath pointers as const since they
didn't expect to make modifications.  However, when compiled with
lockdep the datapath mutex pointer is passed to lockdep_is_held(),
which has a non-const argument.  That provoked warnings about
casting away the const, so this drops the const from the original
pointers.

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