ceph: Remove bogus check in invalidatepage

The early bug checks are moot because the VMA layer ensures those things.

1. It will not call invalidatepage unless PagePrivate (or PagePrivate2) are set
2. It will not call invalidatepage without taking a PageLock first.
3. Guantrees that the inode page is mapped.

Signed-off-by: Milosz Tanski <milosz@adfin.com>
Reviewed-by: Sage Weil <sage@inktank.com>