pnfs/blocklayout: correctly decrement extent length

When we do non-page sized reads we can underflow the extent_length variable
and read incorrect data.  Fix the extent_length calculation and change to
defensive <= checks for the extent length in the read and write path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>