initramfs-framework: unmount automounts before switch_root

If mounts are left lingering, then after we switch_root, attempts to
modify the block devices will result in an EBUSY with no way to unmount
them.  As we're about to switch_root anyways, there isn't much use to
keep anything mounted unless it has the new rootfs.

Signed-off-by: Justin Bronder <jsbronder@cold-front.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 4dc7af6d)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
(cherry picked from commit 99163149)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>