net/mlx5: Support for attaching multiple underlay QPs to root flow table

Previous support allowed connecting only a single QPN to the FT.
Now using a linked list multiple QPNs can be attached to the same FT.

Supporting attaching multiple underlay QPs is required for PKEY
support in which child and parent share the same FT.

The actual attaching/detaching FW commands will be called inside the
function symmetrically.

This change requires a change in IPoIB open and close functions, the
attaching/detaching to/from the FT is done each time we open/close.

Signed-off-by: Alex Vesker <valex@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>