vsock/test: fix SEQPACKET message bounds test

vsock/test: fix SEQPACKET message bounds test

Tune message length calculation to make this test work on machines
where 'getpagesize()' returns >32KB. Now maximum message length is not
hardcoded (on machines above it was smaller than 'getpagesize()' return
value, thus we get negative value and test fails), but calculated at
runtime and always bigger than 'getpagesize()' result. Reproduced on
aarch64 with 64KB page size.

Fixes: 5c338112e48a ("test/vsock: rework message bounds test")
Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reported-by: Bogdan Marcynkov <bmarcynk@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20231121211642.163474-1-avkrasnov@salutedevices.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>