NFS: Fix nfs_migrate_page()

The call to migrate_page() will cause the page->private field to be
cleared.
Also fix up the locking around the page->private transfer, so that we ensure
that calls to nfs_page_find_request() don't end up racing.

Finally, fix up a double free bug: nfs_unlock_request() already calls
nfs_release_request() for us...

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Tested-by: Andi Kleen <andi@firstfloor.org>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>