xfs: fix the forward progress assertion in xfs_iwalk_run_callbacks

commit a5336d6bb2d02d0e9d4d3c8be04b80b8b68d56c8 upstream.

In commit 27c14b5daa82 we started tracking the last inode seen during an
inode walk to avoid infinite loops if a corrupt inobt record happens to
have a lower ir_startino than the record preceeding it.  Unfortunately,
the assertion trips over the case where there are completely empty inobt
records (which can happen quite easily on 64k page filesystems) because
we advance the tracking cursor without actually putting the empty record
into the processing buffer.  Fix the assert to allow for this case.

Reported-by:  <zlang@redhat.com>
Fixes: 27c14b5daa82 ("xfs: ensure inobt record walks always make forward progress")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>