fuzz: for fuzzing the rtrmapbt, find the path to the rt rmap btree file

fuzz: for fuzzing the rtrmapbt, find the path to the rt rmap btree file

The fs population code creates a realtime rmap btree in /some/ realtime
group with at least two levels.  This rmapbt file isn't necessarily the
one for group 0, so we need to find it programmatically.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>