bnxt_en: Fix first buffer size calculations for XDP multi-buffer

bnxt_en: Fix first buffer size calculations for XDP multi-buffer

The size of the first buffer is always page size, and the useable
space is the page size minus the offset and the skb_shared_info size.
Make sure SKB and XDP buf sizes match so that the skb_shared_info
is at the same offset seen from the SKB and XDP_BUF.

build_skb() should be passed PAGE_SIZE.  xdp_init_buff() should
be passed PAGE_SIZE as well.  xdp_get_shared_info_from_buff() will
automatically deduct the skb_shared_info size if the XDP buffer
has frags.  There is no need to keep bp->xdp_has_frags.

Change BNXT_PAGE_MODE_BUF_SIZE to BNXT_MAX_PAGE_MODE_MTU_SBUF
since this constant is really the MTU with ethernet header size
subtracted.

Also fix the BNXT_MAX_PAGE_MODE_MTU macro with proper parentheses.

Fixes: 32861236190b ("bnxt: change receive ring space parameters")
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>