nfs: Remove bogus assignment

Commit 3a6fd1f0 (pnfs/blocklayout: remove read-modify-write handling
in bl_write_pagelist) introduced a bogus assignment pg_index = pg_index
in variable initialization. AFAICS it's just a typo so remove it.
Spotted by Coverity (id 1248711).

CC: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>