Implement .commit_metadata hook for NFS export

In order to implement synchronous NFS metadata semantics ZFS
needs to provide the .commit_metadata hook.  All it takes there
is to make sure changes are committed to ZIL.  Fortunately
zfs_fsync() does just that, so simply calling it from
zpl_commit_metadata() does the trick.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #969

config/kernel-commit-metadata.m4

@@ -0,0 +1,23 @@
+dnl #
+dnl # 2.6.33 API change
+dnl # Added eops->commit_metadata() callback to allow the underlying
+dnl # filesystem to determine the most efficient way to commit the inode.
+dnl # Prior to this the nfs server would issue an explicit fsync().
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_COMMIT_METADATA], [
+	AC_MSG_CHECKING([whether eops->commit_metadata() exists])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/exportfs.h>
+		int commit_metadata(struct inode *inode) { return 0; }
+		static struct export_operations eops __attribute__ ((unused))={
+			.commit_metadata = commit_metadata,
+		};
+	],[
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_COMMIT_METADATA, 1,
+		          [eops->commit_metadata() exists])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -53,6 +53,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_CREATE_UMODE_T
 	ZFS_AC_KERNEL_AUTOMOUNT
 	ZFS_AC_KERNEL_ENCODE_FH_WITH_INODE
+	ZFS_AC_KERNEL_COMMIT_METADATA
 	ZFS_AC_KERNEL_CLEAR_INODE
 	ZFS_AC_KERNEL_INSERT_INODE_LOCKED
 	ZFS_AC_KERNEL_D_MAKE_ROOT