Avoid assertion failure when splitting mbuf cluster.

Avoid assertion failure when splitting mbuf cluster.

m_split() calls m_align() to initialize the data pointer of newly
allocated mbuf.  If the new mbuf will be converted to a cluster,
this is not necessary.  If additionally the new mbuf is larger than
MLEN, this can lead to a panic.
Only call m_align() when a valid m_data is needed.  This is the
case if we do not refecence the existing cluster, but memcpy() the
data into the new mbuf.

Reported-by: syzbot+0e6817f5877926f0e96a@syzkaller.appspotmail.com
OK claudio@ deraadt@