nfs: setattr can only change regular file sizes

The VFS never calls setattr with ATTR_SIZE on anything but regular
files.  Remove the if check and turn it into an assert similar to
what some other file systems do.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>