MAINTAINERS: mark bcachefs externally maintained

As per many long discussion threads, public and private.

Signed-off-by: Linus Torvalds <torbalds@linux-foundation.org>

Author: torvalds

MAINTAINERS

@@ -4216,7 +4216,7 @@ F:	drivers/md/bcache/
 BCACHEFS
 M:	Kent Overstreet <kent.overstreet@linux.dev>
 L:	linux-bcachefs@vger.kernel.org
-S:	Supported
+S:	Externally maintained
 C:	irc://irc.oftc.net/bcache
 P:      Documentation/filesystems/bcachefs/SubmittingPatches.rst
 T:	git https://evilpiepirate.org/git/bcachefs.git