dpif-linux: Implement the API functions to allow multiple handler threads read upcall.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>
dpif-linux: Pass 'struct dpif_linux *' to internal static functions.

This commit reformats the dpif-linux module so that all internal
static functions take 'struct dpif_linux *' as input argument.
This will allow the adding of thread-safety annotations.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>
dpif-linux: Add thread-safety annotations.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>