Permit the fchmod system call.

Fixes bug 22516; bugfix on 0.2.5.4-alpha.

changes/bug22516

@@ -0,0 +1,5 @@
+  o Minor bugfixes (linux seccomp2 sandbox):
+    - Permit the fchmod system call, to avoid crashing on startup when
+      starting with the seccomp2 sandbox and an unexpected set of permissions
+      on the data directory or its contents. Fixes bug 22516; bugfix on
+      0.2.5.4-alpha.

src/common/sandbox.c

@@ -135,6 +135,9 @@ static int filter_nopar_gen[] = {
 #endif
 #ifdef HAVE_PIPE
     SCMP_SYS(pipe),
+#endif
+#ifdef __NR_fchmod
+    SCMP_SYS(fchmod),
 #endif
     SCMP_SYS(fcntl),
     SCMP_SYS(fstat),