bcachefs: Make read_only a mount option again, but hidden

fsck passes read_only as a mount option, and it's required for
nochanges, which it also uses.

Usually read_only is handled by the VFS, but we need to be able to
handle it too; we just don't want to print it out twice, so mark it as a
hidden option.

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