NFSv4.2: Set the correct size scratch buffer for decoding READ_PLUS

The scratch_buf array is 16 bytes, but I was passing 32 to the
xdr_set_scratch_buffer() function. Fix this by using sizeof(), which is
what I probably should have been doing this whole time.

Fixes: d3b00a80

 ("NFS: Replace the READ_PLUS decoding code")
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>