datapath: Prevent linker error of unknown symbol.

datapath: Prevent linker error of unknown symbol.

With the latest change of separating vports into their own modules,
it is necessary to export all public functions in linux/compat/
directory.  Also, we should prefix functions which replace the
upstream ones with 'rpl_' and others with 'ovs_'.  This will prevent
the linker error when vport modules use those functions in the future.
e.g., the to be merged vport-stt module will use the flex_array_*
functions which are not currently exported.

Co-authored-by: Tuan Nguyen <tuan.nguyen@veriksystems.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>