odirect_write.c: Address warning -Wsign-compare

Symptom was:
> tests/odirect_write.c: In function ‘main’:
> tests/odirect_write.c:58:17: error: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]
>    58 |         if (res != buf_size) {
>       |                 ^~

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/odirect_write.c b/tests/odirect_write.c
index 10b63f9..bc9040f 100644
--- a/tests/odirect_write.c
+++ b/tests/odirect_write.c
@@ -55,7 +55,7 @@ int main(int argc, char** argv) {
             perror("failed to write");
             return 1;
         }
-        if (res != buf_size) {
+        if ((size_t)res != buf_size) {
             // Too lazy to write a loop here unless it turns out to be necessary.
             fprintf(stderr, "Failed to write exactly %lu bytes", (unsigned long)amt_read);
         }