net: mctp: test: make cloned_frag buffers more appropriately-sized

net: mctp: test: make cloned_frag buffers more appropriately-sized

In our input_cloned_frag test, we currently allocate our test buffers
arbitrarily-sized at 100 bytes.

We only expect to receive a max of 15 bytes from the socket, so reduce
to a more appropriate size. There are some upcoming changes to the
routing code which hit a frame-size limit on s390, so reduce the usage
before that lands.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20250702-dev-forwarding-v5-2-1468191da8a4@codeconstruct.com.au
Signed-off-by: Paolo Abeni <pabeni@redhat.com>