netfs, mm: Add set/end/wait_on_page_fscache() aliases

Add set/end/wait_on_page_fscache() as aliases of
set/end/wait_page_private_2().  These allow a page to marked with
PG_fscache, the flag to be removed and waiters woken and waiting for the
flag to be cleared.  A ref on the page is also taken and dropped.

[Linus suggested putting the fscache-themed functions into the
 caching-specific headers rather than pagemap.h[1]]

Changes:
v5:
- Mirror the changes to the core routines[2].

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Dave Wysochanski <dwysocha@redhat.com>
Tested-By: Marc Dionne <marc.dionne@auristor.com>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-mm@kvack.org
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel...