ceph: check i_nlink while converting a file handle to dentry

Converting a file handle to a dentry can be done call after the inode
unlink.  This means that __fh_to_dentry() requires an extra check to
verify the number of links is not 0.

The issue can be easily reproduced using xfstest generic/426, which does
something like:

    name_to_handle_at(&fh)
    echo 3 > /proc/sys/vm/drop_caches
    unlink()
    open_by_handle_at(&fh)

The call to open_by_handle_at() should fail, as the file doesn't exist
anymore.

Link: http://tracker.ceph.com/issues/19958
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/export.c

@@ -91,6 +91,10 @@ static struct dentry *__fh_to_dentry(struct super_block *sb, u64 ino)
 		ceph_mdsc_put_request(req);
 		if (!inode)
 			return ERR_PTR(-ESTALE);
+		if (inode->i_nlink == 0) {
+			iput(inode);
+			return ERR_PTR(-ESTALE);
+		}
 	}
 
 	return d_obtain_alias(inode);