xsk: align TX metadata layout across ABIs

Add explicit padding before launch_time so xsk_tx_metadata has the same
layout on 32-bit and 64-bit systems.

On several architectures (csky, i386, nios2, m65k, openrisc, sh), the old
native 32-bit layout put launch_time at offset 12 and had a natural size of
20 bytes. Using sizeof(struct xsk_tx_metadata) as tx_metadata_len was already
rejected because the length must be a multiple of eight, so the
straightforward use of the interface was broken on those ABIs. Userspace
could still register a padded length of 24 bytes, though; mixing the old and
new layouts then silently reads launch_time from the wrong offset and
misprograms packet launch times. This intentionally replaces that
incompatible layout because the affected architectures are unlikely to
have any notable users. (x86_64 and arm64 have the most users and are _not_
affected)

Fixes: ca4419f15abd ("xsk: Add launch time hardware offload support to XDP Tx metadata")
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260819160535.1472459-2-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/if_xdp.h

@@ -149,6 +149,7 @@ struct xsk_tx_metadata {
 			__u16 csum_start;
 			/* Offset from csum_start where checksum should be stored. */
 			__u16 csum_offset;
+			__u32 reserved;
 
 			/* XDP_TXMD_FLAGS_LAUNCH_TIME */
 			/* Launch time in nanosecond against the PTP HW Clock */