fusefs: rewrite vop_getpages and vop_putpages

fusefs: rewrite vop_getpages and vop_putpages

Use the standard facilities for getpages and putpages instead of bespoke
implementations that don't work well with the writeback cache.  This has
several corollaries:

* Change the way we handle short reads _again_.  vfs_bio_getpages doesn't
  provide any way to handle unexpected short reads.  Plus, I found some more
  lock-order problems.  So now when the short read is detected we'll just
  clear the vnode's attribute cache, forcing the file size to be requeried
  the next time it's needed.  VOP_GETPAGES doesn't have any way to indicate
  a short read to the "caller", so we just bzero the rest of the page
  whenever a short read happens.

* Change the way we decide when to set the FUSE_WRITE_CACHE bit.  We now set
  it for clustered writes even when the writeback cache is not in use.

Sponsored by:   The FreeBSD Foundation