vfs: better rcu file deletion

Instead of rcu-disposing the memory in a file object, rcu-dispose the object
itself.  This prevents issues with accessing a destroyed (but not freed)
object.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
pthread: remove magic number for limit of pthread tsd keys

Use PTHREAD_KEYS_MAX (value is 128) instead.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Zifei Tong <zifeitong@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>