drm/v3d: Reject empty multisync extension to prevent infinite loop

v3d_get_extensions() walks a userspace-provided singly-linked list of
ioctl extensions without any bound on the chain length. A local user
can craft a self-referential extension (ext->next == &ext) with zero
in_sync_count and out_sync_count, which bypasses the existing duplicate-
extension guard:

if (se->in_sync_count || se->out_sync_count)
return -EINVAL;

The guard never fires because v3d_get_multisync_post_deps() returns
immediately when count is zero, leaving both fields at zero on every
iteration. The result is an infinite loop in kernel context, blocking
the calling thread and pegging a CPU core indefinitely.

Fix this by rejecting a multisync extension where both in_sync_count
and out_sync_count are zero in v3d_get_multisync_submit_deps(). An
empty multisync carries no synchronization information and serves no
useful purpose, so returning -EINVAL for such an extension is the
correct defense against this attack vector.

Fixes: e4165ae8304e ("drm/v3d: add multiple syncobjs support")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
Link: https://patch.msgid.link/20260415050000.3816128-1-ashutoshdesai993@gmail.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>