ceph: explicitly specify page alignment in network messages

The alignment used for reading data into or out of pages used to be taken
from the data_off field in the message header.  This only worked as long
as the page alignment matched the object offset, breaking direct io to
non-page aligned offsets.

Instead, explicitly specify the page alignment next to the page vector
in the ceph_msg struct, and use that instead of the message header (which
probably shouldn't be trusted).  The alloc_msg callback is responsible for
filling in this field properly when it sets up the page vector.

Signed-off-by: Sage Weil <sage@newdream.net>

include/linux/ceph/messenger.h

@@ -82,6 +82,7 @@ struct ceph_msg {
 	struct ceph_buffer *middle;
 	struct page **pages;            /* data payload.  NOT OWNER. */
 	unsigned nr_pages;              /* size of page array */
+	unsigned page_alignment;        /* io offset in first page */
 	struct ceph_pagelist *pagelist; /* instead of pages */
 	struct list_head list_head;
 	struct kref kref;