net/mlx5e: Allow concurrent creation of mod_hdr entries

Mod_hdr entries creation is fully synchronized by mod_hdr_tbl->lock. In
order to allow concurrent allocation of hardware resources used to offload
header rewrite, extend mlx5e_mod_hdr_entry with 'res_ready' completion.
Move call to mlx5_modify_header_alloc() out of mod_hdr_tbl->lock critical
section. Modify code that attaches new flows to existing mh to wait for
'res_ready' completion before using the entry. Insert mh to mod_hdr table
before provisioning it to hardware and modify all users of mod_hdr table to
verify that mh was fully initialized by checking completion result for
negative value (and to wait for 'res_ready' completion, if necessary).

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>