fixed error reporting when fallocate fails

ChangeLog

@@ -52,6 +52,7 @@
 	* added more detailed instrumentation of the disk I/O thread
 
 
+	* fixed error reporting when fallocate fails
 	* deprecate support for separate proxies for separate kinds of connections
 
 0.15.4 release

src/file.cpp

@@ -1572,7 +1572,7 @@ namespace libtorrent
 			// and report it.
 			if (errno != ENOSYS)
 			{
-				ec.assign(ret, get_posix_category());
+				ec.assign(errno, get_posix_category());
 				return false;
 			}
 #endif // TORRENT_LINUX