nfs: fix nfs_size_to_loff_t

commit 50ab8ec7 upstream.

See http: //www.infradead.org/rpr.html
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We support OFFSET_MAX just fine, so don't round down below it.  Also
switch to using min_t to make the helper more readable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Fixes: 433c9237 ("NFS: Clean up nfs_size_to_loff_t()")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>