tst-async: migration lock is enough

As Nadav pointed out tst-async disabled preemption only to make sure
that two function calls happen on the same CPU. For that purpose migration
lock is sufficient.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
runtime: stub __cxa_thread_atexit_impl

Needed for gcc 4.9.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs: avoid recursive locking on vp when checking vnode refcnt

vcount is used to get vnode refcnt amount, which unnecessarily takes
another lock for the same vnode.
Found it while auditing the code.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>