net: fix signedness bug in skb_splice_from_iter()

The "len" variable needs to be signed for the error handling to work
correctly.

Fixes: 2e910b95 ("net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/366861a7-87c8-4bbf-9101-69dd41021d07@kili.mountain


Signed-off-by: Jakub Kicinski <kuba@kernel.org>