log: Don't modify global @errno in __print_on_level

The __print_on_level routine may modify global @errno
variable which is inacceptable: this is logging routine
which must be transparent to the rest of the program code.

Thus save @errno in local @__errno variable and restore
it on return.

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