NFS: Fix up the fsync code

Christoph points out that the VFS will always flush out data before calling
nfs_fsync(), so we can dispense with a full call to nfs_wb_all(), and
replace that with a simpler call to nfs_commit_inode().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>