hypfs_kill_super(): deal with failed allocations

commit a24cd490739586a7d2da3549a1844e1d7c4f4fc4 upstream.

hypfs_fill_super() might fail to allocate sbi; hypfs_kill_super()
should not oops on that.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>