xfs: use separate lock classes for realtime metadata inode ILOCKs

Realtime metadata files are not quite regular files because userspace
can't access the realtime bitmap directly, and because we take the ILOCK
of the rt bitmap file while holding the ILOCK of a realtime file.  The
double nature of inodes confuses lockdep, so up until now we've created
lockdep subclasses to help lockdep keep things straight.

We've gotten away with using lockdep subclasses because there's only two
rt metadata files, but with the coming addition of realtime rmap and
refcounting, we'd need two more subclasses, which is a lot of class bits
to burn on a side feature.

Therefore, switch to manually setting the lockdep class of the rt
metadata ILOCKs.  In the next patch we'll remove the rt-related ILOCK
subclasses.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>

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