xfs: tidy up corruption reporting

Linus requested that I audit the XFS code base to make sure that we
always log something to dmesg when returning EFSCORRUPTED or EFSBADCRC
to userspace.  These patches are the results of that audit.

The first three patches start by refactoring a few helper methods so
that in the third patch we always report something to dmesg whenever we
decide to return EFSCORRUPTED.