bcachefs: bcachefs_metadata_version_casefolding

bcachefs: bcachefs_metadata_version_casefolding

This patch implements support for case-insensitive file name lookups
in bcachefs.

The implementation uses the same UTF-8 lowering and normalization that
ext4 and f2fs is using.

More information is provided in Documentation/bcachefs/casefolding.rst

Compatibility notes:

This uses the new versioning scheme for incompatible features where an
incompatible feature is tied to a version number: the superblock says
"we may use incompat features up to x" and "incompat features up to x
are in use", disallowing mounting by previous versions.

Additionally, and old style incompat feature bit is used, so that
kernels without utf8 casefolding support know if casefolding
specifically is in use and they're allowed to mount.

Signed-off-by: Joshua Ashton <joshua@froggi.es>
Cc: André Almeida <andrealmeid@igalia.com>
Cc: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>