ns: Fix wrong opened net ns file

Since net ns is assigned after prepare_fds() and,
in common case, at the moment of open_ns_fd() call
task points to a net ns, which differs to its target
net ns, we can't get the ns from a task. So, get it
from fdstore. Also, support userns ns fds.

v2: Add comment

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>