initramfs: cleanup incomplete rootfs

initramfs: cleanup incomplete rootfs

Unpacking an external initrd may fail e.g.  not enough memory.  This
leads to an incomplete rootfs because some files might be extracted
already.  Fixed by cleaning the rootfs so the kernel is not using an
incomplete rootfs.

Link: http://lkml.kernel.org/r/20181030151805.5519-1-david.engraf@sysgo.com
Signed-off-by: David Engraf <david.engraf@sysgo.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>