bcachefs: Fix bch_sb_handle type

blk_mode_t was recently introduced; we should be using it now, instead
of fmode_t.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super_types.h

@@ -8,7 +8,7 @@ struct bch_sb_handle {
 	struct bio		*bio;
 	void			*holder;
 	size_t			buffer_size;
-	fmode_t			mode;
+	blk_mode_t		mode;
 	unsigned		have_layout:1;
 	unsigned		have_bio:1;
 	unsigned		fs_sb:1;