pNFS/flexfiles: Make local symbol layoutreturn_ops static

Fixes the following sparse warning:

fs/nfs/flexfilelayout/flexfilelayout.c:2114:34: warning:
 symbol 'layoutreturn_ops' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>