selftests/net/socket.c: removed warnings from unused returns

socket.c: In function ‘run_tests’:
socket.c:59:25: warning: ignoring return value of ‘strerror_r’ \
declared with attribute ‘warn_unused_result’ [-Wunused-result]
59 | strerror_r(-s->expect, err_string1, ERR_STRING_SZ);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

socket.c:60:25: warning: ignoring return value of ‘strerror_r’ \
declared with attribute ‘warn_unused_result’ [-Wunused-result]
60 | strerror_r(errno, err_string2, ERR_STRING_SZ);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

socket.c:73:33: warning: ignoring return value of ‘strerror_r’ \
declared with attribute ‘warn_unused_result’ [-Wunused-result]
73 | strerror_r(errno, err_string1, ERR_STRING_SZ);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

changelog:
v2
- const char* messages and fixed patch warnings of max 75 chars
  per line

Signed-off-by: Alex Tran <alex.t.tran@gmail.com>
Link: https://patch.msgid.link/20250819025227.239885-1-alex.t.tran@gmail.com


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