afs: Fix afs_launder_page() to set correct start file position

Fix afs_launder_page() to set the starting position of the StoreData RPC at
the offset into the page at which the modified data starts instead of at
the beginning of the page (the iov_iter is correctly offset).

The offset got lost during the conversion to passing an iov_iter into
afs_store_data().

Changes:
ver #2:
 - Use page_offset() rather than manually calculating it[1].

Fixes: bd80d8a8 ("afs: Use ITER_XARRAY for writing")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/YST/0e92OdSH0zjg@casper.infradead.org/ [1]
Link: https://lore.kernel.org/r/162880783179.3421678.7795105718190440134.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/162937512409.1449272.18441473411207824084.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/162981148752.1901565.3663780601682206026.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163005741670.2472992.2073548908229887941.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163221839087.3143591.14278359695763025231.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163292980654.4004896.7134735179887998551.stgit@warthog.procyon.org.uk/ # v2

fs/afs/write.c

@@ -974,8 +974,7 @@ int afs_launder_page(struct page *page)
 		iov_iter_bvec(&iter, WRITE, bv, 1, bv[0].bv_len);
 
 		trace_afs_page_dirty(vnode, tracepoint_string("launder"), page);
-		ret = afs_store_data(vnode, &iter, (loff_t)page->index * PAGE_SIZE,
-				     true);
+		ret = afs_store_data(vnode, &iter, page_offset(page) + f, true);
 	}
 
 	trace_afs_page_dirty(vnode, tracepoint_string("laundered"), page);