NFS: remove redundant code in nfs_file_write()

filemap_fdatawait_range() will always return 0, after patch 6c984083


("NFS: Use of mapping_set_error() results in spurious errors"), it will not
save the wb err in struct address_space->flags:

  result = filemap_fdatawait_range(file->f_mapping, ...) = 0
    filemap_check_errors(mapping) = 0
      test_bit(..., &mapping->flags) // flags is 0

Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>