bnxt: fix crashes when reducing ring count with active RSS contexts

bnxt doesn't check if a ring is used by RSS contexts when reducing
ring count. Core performs a similar check for the drivers for
the main context, but core doesn't know about additional contexts,
so it can't validate them. bnxt_fill_hw_rss_tbl_p5() uses ring
id to index bp->rx_ring[], which without the check may end up
being out of bounds.

  BUG: KASAN: slab-out-of-bounds in __bnxt_hwrm_vnic_set_rss+0xb79/0xe40
  Read of size 2 at addr ffff8881c5809618 by task ethtool/31525
  Call Trace:
  __bnxt_hwrm_vnic_set_rss+0xb79/0xe40
   bnxt_hwrm_vnic_rss_cfg_p5+0xf7/0x460
   __bnxt_setup_vnic_p5+0x12e/0x270
   __bnxt_open_nic+0x2262/0x2f30
   bnxt_open_nic+0x5d/0xf0
   ethnl_set_channels+0x5d4/0xb30
   ethnl_default_set_doit+0x2f1/0x620

Core does track the additional contexts in net-next, so we can
move this validation out of the driver as a follow up there.

Fixes: b3d0083c

 ("bnxt_en: Support RSS contexts in ethtool .{get|set}_rxfh()")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://patch.msgid.link/20240705020005.681746-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>