Enable experimental features. These features may not be stable enough
for end users. This is meant for btrfs developers or users who wish
to test the functionality and report problems.

Current list:

- COW fixup worker warning - last warning before removing the
functionality catching out-of-band page
dirtying, not necessary since 5.8

- RAID mirror read policy - additional read policies for balancing
reading from redundant block group
profiles (currently: pid, round-robin,
fixed devid)

- send stream protocol v3 - fs-verity support

- checksum offload mode - sysfs knob to affect when checksums are
calculated (at IO time, or in a thread)

- raid-stripe-tree - additional mapping of extents to devices to
support RAID1* profiles on zoned devices,
RAID56 not yet supported

- extent tree v2 - complex rework of extent tracking

If unsure, say N.