pnfs/blocklayout: fix last_write_offset incorrectly set to page boundary

Commit 41963c10 sets the block layout's
last written byte to the offset of the end of the extent rather than the
end of the write which incorrectly updates the inode's size for
partial-page writes.

Fixes: 41963c10 ("pnfs/blocklayout: update last_write_offset atomically with extents")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org # 4.8+
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>