[CVE-2017-14340] xfs: XFS_IS_REALTIME_INODE() should be false if no rt device

[Thadeu Lima de Souza Cascardo]

Trusty needed a backport because the macro has been moved between files. Artful
already has a backport, so it's not included in this submission.

Patch has been built for all three series, and tested that it fixes the problem
on Xenial.