add a "discard cache" debug option to zloop

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series adds a new option to zloop to lose data not committed to stable
storage using a flush operation on device removal.  The idea is to help
testing that file system code does the right thing in face of volatile
write caches.  For conventional devices, this can be tested using
dm-log-writes, but the concepts there don't work for sequential write
required zones.  Instead this adds an option to zloop, which records the
write pointer at the last cache flush for each zone file in an xattr,
and truncates the files down to that value on removal, simulating losing
the contents of the volatile write cache.

Diffstat:
 Documentation/admin-guide/blockdev/zoned_loop.rst |    5 
 drivers/block/zloop.c                             |  337 ++++++++++++++--------
 2 files changed, 226 insertions(+), 116 deletions(-)