afs: Fix where page->private is set during write

In afs, page->private is set to indicate the dirty region of a page.  This
is done in afs_write_begin(), but that can't take account of whether the
copy into the page actually worked.

Fix this by moving the change of page->private into afs_write_end().

Fixes: 4343d008 ("afs: Get rid of the afs_writeback record")
Signed-off-by: David Howells <dhowells@redhat.com>