ceph: add fscache writeback support

When updating the backing store from the pagecache (a'la writepage or
writepages), write to the cache first. This allows us to keep caching
files even when they are being written, as long as we have appropriate
caps.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20211129162907.149445-3-jlayton@kernel.org/ # v1
Link: https://lore.kernel.org/r/20211207134451.66296-3-jlayton@kernel.org/ # v2
Link: https://lore.kernel.org/r/163906985808.143852.1383891557313186623.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967190257.1823006.16713609520911954804.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021585020.640689.6765214932458435472.stgit@warthog.procyon.org.uk/ # v4