xfs: metadata inode directories

This series delivers a new feature -- metadata inode directories.  This
is a separate directory tree (rooted in the superblock) that contains
only inodes that contain filesystem metadata.  Different metadata
objects can be looked up with regular paths.

We start by creating xfs_imeta_* functions to mediate access to metadata
inode pointers.  This enables the imeta code to abstract inode pointers,
whether they're the classic five in the superblock, or the much more
complex directory tree.  All current users of metadata inodes (rt+quota)
are converted to use the boilerplate code.

Next, we define the metadir on-disk format, which consists of marking
inodes with a new iflag that says they're metadata.  This we use to
prevent bulkstat and friends from ever getting their hands on fs
metadata.

Finally, we implement metadir operations so that clients can create,
delete, zap, and look up metadata inodes by path.  Beware that much of
this code is only lightly used, because the five current users of
metadata inodes don't tend to change them very often.  This is likely to
change if and when the subvolume and multiple-rt-volume features get
written/merged/etc.