nfs/blocklayout: make sure making a aligned read request

Only treat write goes up to the inode size as aligned request,
because it always write PAGE_CACHE_SIZE, but read a dynamic size.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>