buffer: attempt to size raw_pipe buffers

buffer: attempt to size raw_pipe buffers

Make sure the requested length is below the maximum pipe size for now,
since we're only using one pipe and splicing once into and out of
it. The default max is 1MB on recent kernels, so this isn't such a
terrible limitation.

To get around this we could use multiple pipes, or keep both source and
destination fds open at the same time and call splice many times. This
is more usual usage for splice, but would require a lot more work to
restructure the filestore and messenger to handle it.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>