LU-2900 llite: Null deref in ll_fsync:mkdir on NFSmounted Lus

When a Lustre file system is mounted via NFS and a mkdir
operation is attempted, a null pointer dereference occurs
in ll_fsync.

With 2.x, Lustre added support for different VFS fsync APIs
that do not include a dentry parameter.

To make the logic the same in all cases, the old ll_fsync
interface was changed to pull the inode from f_dentry
in the *file parameter.

In some cases when using the old ll_fsync interface, the
caller does not set the f_dentry part of the *file parameter
resulting in a NULL dereference. The fix to this is to
restore the old logic in those cases: when a dentry
parameter is provided, get the inode from that parameter
rather than the file parameter.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I93ecf04e23121c76e571383d74e2fc902565614e
Reviewed-on: http://review.whamcloud.com/5585
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Cory Spitz <spitzcor@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>