Log the real reason for why posix_fadvise failed (#430)

`reclaimFilePageCache` did not set `errno` but `rdbSaveInternal` which
is logging the error assumed it did. This makes sure `errno` is set.

Signed-off-by: Ted Lyngmo <ted@lyncon.se>

src/util.c

@@ -1138,7 +1138,10 @@ int fsyncFileDir(const char *filename) {
 int reclaimFilePageCache(int fd, size_t offset, size_t length) {
 #ifdef HAVE_FADVISE
     int ret = posix_fadvise(fd, offset, length, POSIX_FADV_DONTNEED);
-    if (ret) return -1;
+    if (ret) {
+        errno = ret;
+        return -1;
+    }
     return 0;
 #else
     UNUSED(fd);