selftests: mptcp: sockopt: fix error messages

selftests: mptcp: sockopt: fix error messages

This patch fixes several issues in the error reporting of the MPTCP sockopt
selftest:

1. Fix diff not printed: The error messages for counter mismatches had
   the actual difference ('diff') as argument, but it was missing in the
   format string. Displaying it makes the debugging easier.

2. Fix variable usage: The error check for 'mptcpi_bytes_acked' incorrectly
   used 'ret2' (sent bytes) for both the expected value and the difference
   calculation. It now correctly uses 'ret' (received bytes), which is the
   expected value for bytes_acked.

3. Fix off-by-one in diff: The calculation for the 'mptcpi_rcv_delta' diff
   was 's.mptcpi_rcv_delta - ret', which is off-by-one. It has been
   corrected to 's.mptcpi_rcv_delta - (ret + 1)' to match the expected
   value in the condition above it.

Fixes: 5dcff89e1455 ("selftests: mptcp: explicitly tests aggregate counters")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-5-40171884ade8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>