goldfish_pipe: fix build warning about using too much stack.

The new goldfish_pipe code used too much stack space in the
transfer_max_buffers() call.  As the function is serialized with a lock,
let's make the buffer static to not use the stack for the large buffer.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Yurii Zubrytskyi <zyy@google.com>
Cc: Jin Qian <jinqian@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>