ceph: extend ceph_writeback_ctl for ceph_writepages_start() refactoring

The ceph_writepages_start() has unreasonably huge size and
complex logic that makes this method hard to understand.
Current state of the method's logic makes bug fix really
hard task. This patch extends the struct ceph_writeback_ctl
with the goal to make ceph_writepages_start() method
more compact and easy to understand by means of deep refactoring.

Signed-off-by: Viacheslav Dubeyko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Tested-by: David Howells <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: dubeyko