net/core/skmsg: Delete an unnecessary check before the function call “consume_skb”

The consume_skb() function performs also input parameter validation.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>