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
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