bcachefs: Initialize super_block->s_uuid

Need to fix this oversight for the new FS_IOC_(GET|SET)UUID ioctls.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -1878,6 +1878,7 @@ static struct dentry *bch2_mount(struct file_system_type *fs_type,
 	sb->s_time_gran		= c->sb.nsec_per_time_unit;
 	sb->s_time_min		= div_s64(S64_MIN, c->sb.time_units_per_sec) + 1;
 	sb->s_time_max		= div_s64(S64_MAX, c->sb.time_units_per_sec);
+	sb->s_uuid		= c->sb.user_uuid;
 	c->vfs_sb		= sb;
 	strscpy(sb->s_id, c->name, sizeof(sb->s_id));