ceph: break out writeback of incompatible snap context to separate function

When dirtying a page, we have to flush incompatible contexts. Move the
search for an incompatible context into a separate function, and fix up
the caller to wait and retry if there is one.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>