xfs: indirect health reporting

This series enables the XFS health reporting infrastructure to remember
indirect health concerns when resources are scarce.  For example, if a
scrub notices that there's something wrong with an inode's metadata but
memory reclaim needs to kill the incore inode, we ought to be able to
report that indirect observation in the AGI health report.

The first two patches in this series set that up, and the third one
provides a means for xfs_scrub to tell the kernel that it can forget the
indirect problem report.
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