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The external log device fsmap backend doesn't have an rmapbt to query,
so it's wasteful to spend time initializing the rmap_irec objects.
Worse yet, the log could (someday) be longer than 2^32 fsblocks, so
using the rmap irec structure will result in integer overflows.

Fix this mess by computing the start address that we want from keys[0]
directly, and use the daddr-based record filtering algorithm that we
also use for rtbitmap queries.

Fixes: e89c0413

 ("xfs: implement the GETFSMAP ioctl")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>