NTB: MFV b77b2637: Link toggle memory leak

Each link-up will allocate a new NTB receive buffer when the NTB
properties are negotiated with the remote system. These allocations did
not check for existing buffers and thus did not free them. Now, the
driver will check for an existing buffer and free it if not of the
correct size, before trying to alloc a new one.

Authored by: Jon Mason
Obtained from: Linux
Sponsored by: EMC / Isilon Storage Division