bcachefs: BCH_INODE_has_case_insensitive

bcachefs: BCH_INODE_has_case_insensitive

Add a flag for tracking whether a directory has case-insensitive
descendents - so that overlayfs can disallow mounting, even though the
filesystem supports case insensitivity.

This is a new on disk format version, with a (cheap) upgrade to ensure
the flag is correctly set on existing inodes.

Create, rename and fssetxattr are all plumbed to ensure the new flag is
set, and we've got new fsck code that hooks into check_inode(0.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>