fscache, cachefiles: Add alternate API to use kiocb for read/write to cache

fscache, cachefiles: Add alternate API to use kiocb for read/write to cache

Add an alternate API by which the cache can be accessed through a kiocb,
doing async DIO, rather than using the current API that tells the cache
where all the pages are.

The new API is intended to be used in conjunction with the netfs helper
library.  A filesystem must pick one or the other and not mix them.

Filesystems wanting to use the new API must #define FSCACHE_USE_NEW_IO_API
before #including the header

Signed-off-by: David Howells <dhowells@redhat.com>