libceph: hold off building osd request

Defer building the osd request until just before submitting it in
all callers except ceph_writepages_start().  (That caller will be
handed in the next patch.)

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 3d6dcf2..47826c2 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -540,9 +540,6 @@
 	if (IS_ERR(req))
 		return PTR_ERR(req);
 
-	ceph_osdc_build_request(req, pos, num_ops, ops,
-				snapc, vino.snap, &mtime);
-
 	/* write from beginning of first page, regardless of io alignment */
 	page_align = file->f_flags & O_DIRECT ? buf_align : io_align;
 	num_pages = calc_pages_for(page_align, len);
@@ -583,6 +580,10 @@
 	req->r_data_out.alignment = page_align;
 	req->r_inode = inode;
 
+	/* BUG_ON(vino.snap != CEPH_NOSNAP); */
+	ceph_osdc_build_request(req, pos, num_ops, ops,
+				snapc, vino.snap, &mtime);
+
 	ret = ceph_osdc_start_request(&fsc->client->osdc, req, false);
 	if (!ret) {
 		if (req->r_safe_callback) {