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: bea1412 ("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>

Author: e.kubanski

include/net/xsk_buff_pool.h

@@ -240,8 +240,8 @@ static inline u64 xp_get_handle(struct xdp_buff_xsk *xskb,
 		return orig_addr;
 
 	offset = xskb->xdp.data - xskb->xdp.data_hard_start;
-	orig_addr -= offset;
 	offset += pool->headroom;
+	orig_addr -= offset;
 	return orig_addr + (offset << XSK_UNALIGNED_BUF_OFFSET_SHIFT);
 }