datapath: Backport flex_arrays.

flex_arrays didn't exist at all until 2.6.30, weren't exported to modules
until 2.6.38, and performed poorly until 3.0, so this backports the
functionality to older kernels.

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