util: Extend reopen_fd_as_safe

Our loggin engine prints file:line only at invoke point,
so it's unable to see where exactly reopen_fd_as_safe is
failed.

With this patch the output is more human readable

 | Error (util.c:96): fd 7 already in use (called at files.c:359)

Ideally we need full backtrace here, but it's different task.

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