linux: fsync on a directory should return 0, not EINVAL

Directory writes are synchronous, so this is fine. There's a
mostly-convenient function in fs/libfs.c that returns 0 that we can use
to do what we want ("mostly" because it was renamed in 2.6.35).

FIXES 130425

Reviewed-on: http://gerrit.openafs.org/6491
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 267934d0e6910c8d8166a6e78f93c1bab40857b8)

Change-Id: Iaeb8a699673b6144c186b470f6d877fb54f1e319
Reviewed-on: http://gerrit.openafs.org/6493
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>

acinclude.m4

@@ -896,6 +896,9 @@ case $AFS_SYSNAME in *_linux* | *_umlinux*)
 		 AC_CHECK_LINUX_FUNC([zero_user_segments],
 				     [#include <linux/highmem.h>],
 				     [zero_user_segments(NULL, 0, 0, 0, 0);])
+		 AC_CHECK_LINUX_FUNC([noop_fsync],
+				     [#include <linux/fs.h>],
+				     [noop_fsync(NULL, 0, 0, 0);])
 
 		 dnl Consequences - things which get set as a result of the
 		 dnl                above tests

src/afs/LINUX/osi_vnodeops.c

@@ -655,6 +655,11 @@ struct file_operations afs_dir_fops = {
   .open =	afs_linux_open,
   .release =	afs_linux_release,
   .llseek =	default_llseek,
+#ifdef HAVE_LINUX_NOOP_FSYNC
+  .fsync =	noop_fsync,
+#else
+  .fsync =	simple_sync_file,
+#endif
 };
 
 struct file_operations afs_file_fops = {