NFSv4: Fix an Oops in the synchronous write path

 - Missing initialisation of attribute bitmask in _nfs4_proc_write()
 - On success, _nfs4_proc_write() must return number of bytes written.
 - Missing post_op_update_inode() in _nfs4_proc_write()
 - Missing initialisation of attribute bitmask in _nfs4_proc_commit()
 - Missing post_op_update_inode() in _nfs4_proc_commit()

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