mxge: Limit MTU to ~4K

Various code for "gather" jumbo frame is just broken; limiting MTU to 4K
(MJUMPAGESIZE) should workaround this.
mxge: Fix big buf allocation

- MB_WAIT should be used to allocate mbuf.
- m_freem() instead of m_free() should be called if busdma error happens.
- During initialization, there is no mbuf to be recycled; just return
  error.