xsk: discard zero length descriptors in Tx path

Descriptors with zero length are not supported by many NICs. To preserve
uniform behavior discard any zero length desc as invvalid desc.

Signed-off-by: Tirthendu Sarkar <[e-mail]>
Signed-off-by: Maciej Fijalkowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

net/xdp/xsk_queue.h

···
 {
 	u64 offset = desc->addr & (pool->chunk_size - 1);
 
+	if (!desc->len)
+		return false;
+
 	if (offset + desc->len > pool->chunk_size)
 		return false;
 
···
 					      struct xdp_desc *desc)
 {
 	u64 addr = xp_unaligned_add_offset_to_addr(desc->addr);
+
+	if (!desc->len)
+		return false;
 
 	if (desc->len > pool->chunk_size)
 		return false;