fuzzy: for fuzzing ag btrees, find the path to the AG header

fuzzy: for fuzzing ag btrees, find the path to the AG header

The fs population code creates various btrees in /some/ allocation group
with at least two levels.  These btrees aren't necessarily created in
agno 0, so we need to find it programmatically.  While we're at it, fix
a few of the comments that failed to mention when we're fuzzing interior
nodes and not leaves.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>