gfs2: Don't leave s_fs_info pointing to freed memory in init_sbd

When alloc_percpu() fails, sdp gets freed but sb->s_fs_info still points
to the same address. Move the assignment after that error check so that
s_fs_info can only point to a valid sdp or NULL, which is checked for
later in the error path, in gfs2_kill_super().

Reported-by:  <syzbot+dcb8b3587445007f5808@syzkaller.appspotmail.com>
Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>