sfc: allocate a big enough SKB for loopback selftest packet

Cited commits passed a size to alloc_skb that was only big enough for
 the actual packet contents, but the following skb_put + memcpy writes
 the whole struct efx_loopback_payload including leading and trailing
 padding bytes (which are then stripped off with skb_pull/skb_trim).
This could cause an skb_over_panic, although in practice we get saved
 by kmalloc_size_roundup.
Pass the entire size we use, instead of the size of the final packet.

Reported-by: Andy Moreton <andy.moreton@amd.com>
Fixes: cf60ed46 ("sfc: use padding to fix alignment in loopback test")
Fixes: 30c24dd8 ("sfc: siena: use padding to fix alignment in loopback test")
Fixes: 1186c6b3

 ("sfc: falcon: use padding to fix alignment in loopback test")
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230821180153.18652-1-edward.cree@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>