RDMA/iwpm: Delete unnecessary checks before the macro call "dev_kfree_skb"

The dev_kfree_skb() function performs also input parameter validation.
Thus the test around the shown calls is not needed.

This issue was detected by using the Coccinelle software.

Link: https://lore.kernel.org/r/16df4c50-1f61-d7c4-3fc8-3073666d281d@web.de
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>