net/mlx5e: Export sharing of mod headers to a new file

Refactor sharing of mod headers to new file and while there,
remove spin lock and flows list, as this is only used for warn on.

Use the generic API in the next patch to re-use tuple modify headers
for identical modify actions,

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>