ice: fix incorrect counter for buffer allocation failures

Currently, the driver increments `alloc_page_failed` when buffer allocation fails
in `ice_clean_rx_irq()`. However, this counter is intended for page allocation
failures, not buffer allocation issues.

This patch corrects the counter by incrementing `alloc_buf_failed` instead,
ensuring accurate statistics reporting for buffer allocation failures.

Fixes: 2fba7dc5 ("ice: Add support for XDP multi-buffer on Rx side")
Reported-by: Jacob Keller <jacob.e.keller@intel.com>
Suggested-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Michal Kubiak <michal.kubiak@intel.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Priya Singh <priyax.singh@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>