Now, btrfs-progs has a kernel scrub equivalent.
A new option, --offline is added to "btrfs scrub start".

If --offline is given, btrfs scrub will just act like kernel scrub, to
check every copy of extent and do a report on corrupted data and if it's
recoverable.

The advantage compare to kernel scrub is:
1) No race
   Unlike kernel scrub, which is done in parallel, offline scrub is done
   by a single thread.
   Although it may be slower than kernel one, it's safer and no false
   alert.

2) Correctness
   Kernel has a known bug (fix submitted) which will recovery RAID5/6
   data but screw up P/Q, due to the hardness coding in kernel.
   While in btrfs-progs, no page, (almost) no memory size limit, we're
   can focus on the scrub, and make things easier.

New offline scrub can detect and report P/Q corruption with
recoverability report, while kernel will only report data stripe error.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Su <suy.fnst@cn.fujitsu.com>