NFS41/flexfiles: update inode after write finishes

Otherwise we break fstest case tests/read_write/mctime.t

Does files layout need the same fix as well?

Cc: stable@vger.kernel.org # v4.0+
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>