pnfs/blocklayout: reject pnfs blocksize larger than page size

The Linux VM subsystem can't support block sizes larger than page size
for block based filesystems very well.  While this can be hacked around
to some extent for simple filesystems the read-modify-write cycles
required for pnfs block invalid extents are extremly deadlock prone
when operating on multiple pages.  Reject this case early on instead
of pretending to support it (badly).

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