xsk: Fix offset calculation in unaligned mode

Bring back previous offset calculation behaviour
in AF_XDP unaligned umem mode.

In unaligned mode, upper 16 bits should contain
data offset, lower 48 bits should contain
only specific chunk location without offset.

Remove pool->headroom duplication into 48bit address.

Signed-off-by: Eryk Kubanski <e.kubanski@partner.samsung.com>
Fixes: bea14124 ("xsk: Get rid of xdp_buff_xsk::orig_addr")
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://patch.msgid.link/20250416112925.7501-1-e.kubanski@partner.samsung.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>