Move tunnel offloading related code to a separate source file for better
code maintainability.

Code refactoring with no functional change.
Signed-off-by: Oz Shlomo <ozsh@mellanox.com>
Reviewed-by: Eli Britstein <elibr@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>