virtio_balloon: Remove redundant __GFP_NOWARN

virtio_balloon: Remove redundant __GFP_NOWARN

Commit 16f5dfbc851b ("gfp: include __GFP_NOWARN in GFP_NOWAIT")
made GFP_NOWAIT implicitly include __GFP_NOWARN.

Therefore, explicit __GFP_NOWARN combined with GFP_NOWAIT
(e.g., `GFP_NOWAIT | __GFP_NOWARN`) is now redundant. Let's clean
up these redundant flags across subsystems.

No functional changes.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Message-Id: <20250807132643.546237-1-rongqianfeng@vivo.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>