ceph: make ceph_msg_new return NULL on failure; clean up, fix callers

Returning ERR_PTR(-ENOMEM) is useless extra work.  Return NULL on failure
instead, and fix up the callers (about half of which were wrong anyway).

Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 834d2b8..b042609 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -649,8 +649,8 @@
 				    do_sync,
 				    ci->i_truncate_seq, ci->i_truncate_size,
 				    &mtime, false, 2);
-	if (IS_ERR(req))
-		return PTR_ERR(req);
+	if (!req)
+		return -ENOMEM;
 
 	num_pages = calc_pages_for(pos, len);