net: devmem: ksft: upgrade rx test to send 1K data

The current test just sends "hello\nworld" and verifies that is the
string received on the RX side. That is fine, but improve the test a bit
by sending 1K data. The test should be improved further to send more
data, but for now this should be a welcome improvement.

The test will send a repeating pattern of 0x01, 0x02, ... 0x06. The
ncdevmem `-v 7` flag will verify this pattern. ncdevmem will provide
useful debugging info when the test fails, such as the frags received
and verified fine, and which frag exactly failed, what was the expected
byte pattern, and what is the actual byte pattern received. All this
debug information will be useful when the test fails.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250523230524.1107879-8-almasrymina@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>