datapath: Use __read_mostly annotations where appropriate.

datapath: Use __read_mostly annotations where appropriate.

Variables which are changed only infrequently should be annotated
with __read_mostly, which will group them together in a special
linker section.  This prevents them from sharing cache lines with
data on the hot path.

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