libceph: tweak ceph_alloc_msg()

The function ceph_alloc_msg() is only used to allocate a message
that will be assigned to a connection's in_msg pointer.  Rename the
function so this implied usage is more clear.

In addition, make that assignment inside the function (again, since
that's precisely what it's intended to be used for).  This allows us
to return what is now provided via the passed-in address of a "skip"
variable.  The return type is now Boolean to be explicit that there
are only two possible outcomes.

Make sure the result of an ->alloc_msg method call always sets the
value of *skip properly.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
diff --git a/net/ceph/mon_client.c b/net/ceph/mon_client.c
index ab6b24a..8462cce 100644
--- a/net/ceph/mon_client.c
+++ b/net/ceph/mon_client.c
@@ -442,6 +442,7 @@
 		m = NULL;
 	} else {
 		dout("get_generic_reply %lld got %p\n", tid, req->reply);
+		*skip = 0;
 		m = ceph_msg_get(req->reply);
 		/*
 		 * we don't need to track the connection reading into
@@ -982,6 +983,8 @@
 	case CEPH_MSG_MDS_MAP:
 	case CEPH_MSG_OSD_MAP:
 		m = ceph_msg_new(type, front_len, GFP_NOFS, false);
+		if (!m)
+			return NULL;	/* ENOMEM--return skip == 0 */
 		break;
 	}