replace posix_fallocate with fallocate

posix_fallocate does not return -1 for an error case.

Replace posix_fallocate for fallocate which does return -1 when
something goes wrong. See fallocate(2):

 On success, fallocate() returns zero.
 On error, -1 is returned and errno is set to indicate the error.

src/main.c

@@ -7,6 +7,7 @@
  * (at your option) any later version.
  */
 
+#define _GNU_SOURCE
 #include <stdlib.h>
 #include <stdio.h>
 #include <ev.h>
@@ -273,7 +274,7 @@ static int tip_client_head_hdr(struct tip_client *cli)
 		return -1;
 	}
 
-	if (posix_fallocate(cli->fd, 0, cli->content_len) < 0) {
+	if (fallocate(cli->fd, 0, 0, cli->content_len) < 0) {
 		syslog(LOG_ERR, "failed to allocate room for file %s: %s",
 		       filename, strerror(errno));
 		return -1;