ceph: fix uninitialized dentry pointer in ceph_real_mount()

fs/ceph/super.c: In function ‘ceph_real_mount’:
fs/ceph/super.c:818: warning: ‘root’ may be used uninitialized in this function

If s_root is already valid, dentry pointer root is never initialized,
and returned by ceph_real_mount(). This will cause a crash later when
the caller dereferences the pointer.

Fixes: ce2728aaa82bbeba ("ceph: avoid accessing / when mounting a subpath")
Signed-off-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Yan, Zheng <[e-mail]>

fs/ceph/super.c

···
 		err = ceph_fs_debugfs_init(fsc);
 		if (err < 0)
 			goto fail;
+	} else {
+		root = dget(fsc->sb->s_root);
 	}
 
 	fsc->mount_state = CEPH_MOUNT_MOUNTED;