xfs: discontiguous multi-block buffer logging test

xfs: discontiguous multi-block buffer logging test

XFS had a bug in the multi-block buffer logging code that caused a
NULL lv panic at log push time due to invalid regions being set in
the buffer log format bitmap. This was demonstrated by modifying a
multi-block directory buffer in a manner that only logs regions
beyond the first FSB-sized mapping of the buffer.

To recreate these conditions, this test fragments free space and
populates several directories with enough entries to require
discontiguous multi-block buffers. To recreate the problem, we
remove entries from the tail end of the directory and fsync to flush
the log.

Note that this test causes a panic on kernels affected by the bug.
As such, it is included in the 'dangerous' group. The bug is
resolved by kernel commit a3916e528b91 ("xfs: fix broken multi-fsb
buffer logging").

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>