xfs: realtime reverse-mapping support

This is the latest revision of a patchset that adds to XFS kernel
support for reverse mapping for the realtime device.  This time around
I've fixed some of the bitrot that I've noticed over the past few
months, and most notably have converted rtrmapbt to use the metadata
inode directory feature instead of burning more space in the superblock.

At the beginning of the set are patches to implement storing B+tree
leaves in an inode root, since the realtime rmapbt is rooted in an
inode, unlike the regular rmapbt which is rooted in an AG block.
Prior to this, the only btree that could be rooted in the inode fork
was the block mapping btree; if all the extent records fit in the
inode, format would be switched from 'btree' to 'extents'.

The next few patches widen the reverse mapping routines to fit the
64-bit numbers required to store information about the realtime
device and establish a new b+tree type (rtrmapbt) for the realtime
variant of the rmapbt.  After that are a few patches to handle rooting
the rtrmapbt in a specific inode that's referenced by the superblock.

Finally, there are patches to implement GETFSMAP with the rtrmapbt and
scrub functionality for the rtrmapbt and rtbitmap; and then wire up the
online scrub functionality.