i40e: use xdp.frame_sz as XDP RxQ info frag_size

The only user of frag_size field in XDP RxQ info is
bpf_xdp_frags_increase_tail(). It clearly expects whole buffer size instead
of DMA write size. Different assumptions in i40e driver configuration lead
to negative tailroom.

Set frag_size to the same value as frame_sz in shared pages mode, use new
helper to set frag_size when AF_XDP ZC is active.

Fixes: a045d2f2d03d ("i40e: set xdp_rxq_info::frag_size")
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://patch.msgid.link/20260305111253.2317394-7-larysa.zaremba@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>