ice: store VF pointer instead of VF ID

The VSI structure contains a vf_id field used to associate a VSI with a
VF. This is used mainly for ICE_VSI_VF as well as partially for
ICE_VSI_CTRL associated with the VFs.

This API was designed with the idea that VFs are stored in a simple
array that was expected to be static throughout most of the driver's
life.

We plan on refactoring VF storage in a few key ways:

  1) converting from a simple static array to a hash table
  2) using krefs to track VF references obtained from the hash table
  3) use RCU to delay release of VF memory until after all references
     are dropped

This is motivated by the goal to ensure that the lifetime of VF
structures is accounted for, and prevent various use-after-free bugs.

With the existing vsi->vf_id, the reference tracking for VFs would
become somewhat convoluted, because each VSI maintains a vf_id field
which will then require performing a look up. This means all these flows
will require reference tracking and proper usage of rcu_read_lock, etc.

We know that the VF VSI will always be backed by a valid VF structure,
because the VSI is created during VF initialization and removed before
the VF is destroyed. Rely on this and store a reference to the VF in the
VSI structure instead of storing a VF ID. This will simplify the usage
and avoid the need to perform lookups on the hash table in the future.

For ICE_VSI_VF, it is expected that vsi->vf is always non-NULL after
ice_vsi_alloc succeeds. Because of this, use WARN_ON when checking if a
vsi->vf pointer is valid when dealing with VF VSIs. This will aid in
debugging code which violates this assumption and avoid more disastrous
panics.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>