btrfs: warn about RAID5/6 being experimental at mount time

btrfs: warn about RAID5/6 being experimental at mount time

Bug-Debian: https://bugs.debian.org/863290
Origin: https://bugs.debian.org/863290#5
Forwarded: https://lore.kernel.org/linux-btrfs/4105665.mVaztBssJx@bagend/

Too many people come complaining about losing their data -- and indeed,
there's no warning outside a wiki and the mailing list tribal knowledge.
Message severity chosen for consistency with XFS -- "alert" makes dmesg
produce nice red background which should get the point across.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
[bwh: Also add_taint() so this is flagged in bug reports]
[2023-01-10: still accurate according to btrfs-progs own manpage:
https://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git/commit/?id=922797e15590b836e377d6dc47b828356cafc2a9]
[2024-03-17: still accurate; manpage is now in Documentation/btrfs-man5.rst
implementation went from disk-io.c to super.c; forwarded the issue]

Gbp-Pq: Topic debian
Gbp-Pq: Name btrfs-warn-about-raid5-6-being-experimental-at-mount.patch