init: fix sysfs mount arguments

It is not good idea to pass empty 'source' argument to  mount(2) because
libmount complains about incorrect /proc/self/mountinfo structure. This
affects many applications such as findmnt, umount and etc.

Let's add fake source argument to sysfs mount command as we do with all other
filesystems.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Signed-off-by: Will Deacon <will.deacon@arm.com>