files-reg: Use global mutex to serialize ghost file creation

Otherwise there is a race between files with same names:

link(name -> ghost)                link(name->ghost)
open(name)
unlink(name)
                                   open(name) -> ENOENT

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>