xfs: help mkfs shed its AG initialization code

In this series, we start by adapting the libxfs AG construction code to
be aware that an internal log can be placed with an AG that is being
initialized.  This is necessary to refactor mkfs to use the AG
construction set instead of its own open-coded initialization work.

In userspace, the next thing we have to do is to fix the uncached buffer
code so that libxfs_putbuf won't try to suck them into the buffer cache;
and then fix delwri_{queue,submit} so that IO errors are returned by the
submit function.

The final patch in the xfsprogs series replaces all of mkfs' AG
initialization functions with a single call to the functions in libxfs.