xfs: fix AGFL extent count calculation in xrep_agfl_fill

xfs: fix AGFL extent count calculation in xrep_agfl_fill

In xrep_agfl_fill(), the call to xagb_bitmap_set() passes
'agbno - 1' as the length argument. However, xagb_bitmap_set()
expects a length (number of blocks), not an end block number.
Passing 'agbno - 1' causes used_extents to record an incorrect
range.

Fix this by calculating the correct length as 'agbno - start',
which represents the actual number of blocks filled into the AGFL.

Signed-off-by: jiazhenyuan <jiazhenyuan@uniontech.com>
Fixes: 014ad53732d2ba ("xfs: use per-AG bitmaps to reap unused AG metadata blocks during repair")
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>