[Nbd] Slight increase to BUFSIZE

[Kurt Fitzner <kfitzner@...75...>]

When the buffer overwrite bug was fixed, the fix was added to check to
see if the block + header was larger than the buffer, instead of making
the buffer to include the header.  The problem is, this has subsequently
precluded requests from being the full 1024k in size.

I would like to request the buffer size be increased to 1024k +
sizeof(struct nbd_reply).  The 20 byte increase is negligible, but will
be a significant help in my case, where my transaction sizes need to be
a power of two.

Also, I'd suggest the buffer not be drawn from the stack.  The default
one meg BUFSIZE is already a significant percentage of the entire stack
size in many kernels.  If someone naively increases it further without
realizing it isn't malloced, it could cause problems.

Regards,

    Kurt Fitzner