It will allow us to find a free pid and will speed up
searching an item by pid.

The current algorithm to searching a free pid may return
a value bigger that kernel.max_pid.

v2: add a coment before insert_pid()
    rename root_rb to pid_root_rb and make it "static"
v3: rename insert_{pid,item} into lookup_create_{pid,item}
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>