xfs: rework online repair incore bitmap

In this series, we make some changes to the incore bitmap code: First,
we shorten the prefix to 'xbitmap'.  Then, we rework some utility
functions for later use by online repair and clarify how the walk
functions are supposed to be used.

Finally, we use all these new pieces to convert the incore bitmap to use
an interval tree instead of linked lists.  This lifts the limitation
that callers had to be careful not to set a range that was already set;
and gets us ready for the btree rebuilder functions needing to be able
to set bits in a bitmap and generate maximal contiguous extents for the
set ranges.
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