evm: make rootfs a special case

Both the EVM HMAC and signature xattr formats are file system
specific and can not be copied from one filesystem to another.

EVM differentiates files without any xattrs (INTEGRITY_UNKNOWN)
from those having protected xattrs (INTEGRITY_NOLABEL).  This
patch treats the rootfs filesystem as a special case, returning
INTEGRITY_UNKNOWN.

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