ice: Add a boundary check in ice_xsk_umem()

In ice_xsk_umem(), variable qid which is later used as an array index,
is not validated for a possible boundary exceedance. Because of that,
a calling function might receive an invalid address, which causes
general protection fault when dereferenced.

To address this, add a boundary check to see if qid is greater than the
size of a UMEM array. Also, don't let user change vsi->num_xsk_umems
just by trying to setup a second UMEM if its value is already set up
(i.e. UMEM region has already been allocated for this VSI).

While at it, make sure that ring->zca.free pointer is always zeroed out
if there is no UMEM on a specified ring.

Signed-off-by: Krzysztof Kazimierczak <krzysztof.kazimierczak@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>