nfsd: minor nfsd_setattr cleanup

commit 758e99fefe1d9230111296956335cd35995c0eaf upstream.

Simplify exit paths, size_change use.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>