kcm: Fix splice support

Flags passed in for splice() syscall should not end up in
skb_recv_datagram(). As SPLICE_F_NONBLOCK == MSG_PEEK, kernel gets
confused: skb isn't unlinked from a receive queue, while strp_msg::offset
and strp_msg::full_len are updated.

Unbreak the logic a bit more by mapping both O_NONBLOCK and
SPLICE_F_NONBLOCK to MSG_DONTWAIT. This way we align with man splice(2) in
regard to errno EAGAIN:

   SPLICE_F_NONBLOCK was specified in flags or one of the file descriptors
   had been marked as nonblocking (O_NONBLOCK), and the operation would
   block.

Fixes: 5121197e ("kcm: close race conditions on sk_receive_queue")
Fixes: 91687355 ("kcm: Splice support")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20250725-kcm-splice-v1-1-9a725ad2ee71@rbox.co


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