xfs: refactor corruption checking and reporting

In this second series, we refactor the code that XFS uses to determine
that it is dealing with corrupt metadata and report that to userspace.

As usual, the first 4 patches perform some small cleanups to lay the
groundwork for the meat of the series, which are in the last two
patches.

The fifth patch replaces the XFS_WANT_CORRUPT* macros with open-coded
versions because it's a little strange that a thing that looks like a
simple function call actually has series effects on code flow.

The sixth patch cleans up all the "if (bad) { XFS_ERROR_REPORT..." code
by combining that into a single XFS_CORRUPT_ON macro that does all that
logging.  This cleans up the error handling code blocks some more.