xfs: clean up the rtbitmap fsmap backend

The rtbitmap fsmap backend doesn't query the rmapbt, so it's wasteful to
spend time initializing the rmap_irec objects.  Worse yet, the logic to
query the rtbitmap is spread across three separate functions, which is
unnecessarily difficult to follow.

Compute the start rtextent that we want from keys[0] directly and
combine the functions to avoid passing parameters around everywhere, and
consolidate all the logic into a single function.  At one point many
years ago I intended to use __xfs_getfsmap_rtdev as the launching point
for realtime rmapbt queries, but this hasn't been the case for a long
time.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>