ima: include rootfs (tmpfs) in ima_appraise_tcb policy

When rootfs supports extended attributes and CONFIG_IMA_APPRAISE_ROOTFS
is enabled, appraise the xattrs.

Changelog v1:
- limit appraising tmpfs to rootfs
- define new IMA_APPRAISE_ROOTFS Kconfig option (based on Josh Boyer's
comment).

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>