gfs2: Don't clear sb->s_fs_info in gfs2_sys_fs_add

When gfs2_sys_fs_add() fails, it sets sb->s_fs_info to NULL on its error
path (see commit 0d515210 ("GFS2: Add kobject release method")).
The intention seems to be to prevent dereferencing sb->s_fs_info once
the object pointed to has been deallocated, but that would be better
achieved by setting the pointer to NULL in free_sbd().

As a consequence, when the call to gfs2_sys_fs_add() fails in
gfs2_fill_super(), sdp = GFS2_SB(inode) will evaluate to NULL in iput()
-> gfs2_drop_inode(), and accessing sdp->sd_flags will be a NULL pointer
dereference.

Fix that by only setting sb->s_fs_info to NULL when actually freeing the
object pointed to in free_sbd().

Fixes: ae9f3bd8 ("gfs2: replace sd_aspace with sd_inode")
Reported-by:  <syzbot+b12826218502df019f9d@syzkaller.appspotmail.com>
Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>