NFS: fix the return value of nfs_file_fsync()

By the commit af7fa165 2010-08-03 NFS: Fix up the fsync code
close(2) became returning the non-zero value even if it went well.
nfs_file_fsync() should return 0 when "status" is positive.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>