nfs: Panic when commit fails

Actually pass the NFS_FILE_SYNC option to the server to avoid a
Panic in nfs_direct_write_complete() when a commit fails.

At the end of an nfs write, if the nfs commit fails, all the writes
will be rescheduled.  They are supposed to be rescheduled as NFS_FILE_SYNC
writes, but the rpc_task structure is not completely intialized and so
the option is not passed.  When the rescheduled writes complete, the
return indicates that they are NFS_UNSTABLE and we try to do another
commit.  This leads to a Panic because the commit data structure pointer
was set to null in the initial (failed) commit attempt.

Signed-off-by: Terry Loftin <terry.loftin@hp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>