ceph: Remove ceph_writepage()

ceph: Remove ceph_writepage()

Ceph already has a writepages operation which is preferred over writepage
in all situations except for page migration.  By adding a migrate_folio
operation, there will be no situations in which ->writepage should
be called.  filemap_migrate_folio() is an appropriate operation to use
because the ceph data stored in folio->private does not contain any
reference to the memory address of the folio.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250217185119.430193-2-willy@infradead.org
Tested-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>