xfs: bmap log intent cleanups

The next major target of online repair are metadata that are persisted
in blocks mapped by a file fork.  In other words, we want to repair
directories, extended attributes, symbolic links, and the realtime free
space information.  For file-based metadata, we assume that the space
metadata is correct, which enables repair to construct new versions of
the metadata in a temporary file.  We then need to swap the file fork
mappings of the two files atomically.  With this patchset, we begin
constructing such a facility based on the existing bmap log items and a
new extent swap log item.

This series cleans up a few parts of the file block mapping log intent
code before we start adding support for realtime bmap intents.  Most of
it involves cleaning up tracepoints so that more of the data extraction
logic ends up in the tracepoint code and not the tracepoint call site,
which should reduce overhead further when tracepoints are disabled.
There is also a change to pass bmap intents all the way back to the bmap
code instead of unboxing the intent values and re-boxing them after the
_finish_one function completes.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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