selftests/landlock: Fix readlink check

The audit_init_filter_exe() helper incorrectly checks the readlink(2)
error because an unsigned integer is used to store the result. Use a
signed integer for this check.

Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]
Fixes: 6a500b22971c ("selftests/landlock: Add tests for audit flags and domain IDs")
Reviewed-by: Günther Noack <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mickaël Salaün <[e-mail]>

tools/testing/selftests/landlock/audit.h

···
 	/* It is assume that there is not already filtering rules. */
 	filter->record_type = AUDIT_EXE;
 	if (!path) {
-		filter->exe_len = readlink("/proc/self/exe", filter->exe,
-					   sizeof(filter->exe) - 1);
-		if (filter->exe_len < 0)
+		int ret = readlink("/proc/self/exe", filter->exe,
+				   sizeof(filter->exe) - 1);
+		if (ret < 0)
 			return -errno;
 
+		filter->exe_len = ret;
 		return 0;
 	}