With the new fscache API, the PageFsCache bit now indicates that the
page is being written to the cache and shouldn't be modified or released
until it's finished.

Change releasepage and invalidatepage to wait on that bit before
returning.

Also define FSCACHE_USE_NEW_IO_API so that we opt into the new fscache
API.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>