kernel - TMPFS - Bug fixing pass - paging to/from swap, vnode recycling
* vm_page_set_validdirty() needs to flag the underlying object or
the vfsync() code for the vnode will not know that its dirty.
* Any paging activity has to sync to swap because the kernel marks the
VM pages as clean afterwords even if we [re]dirty them during the I/O.
Testing the uio for UIO_NOCOPY is sufficient to detect when a tmpfs_write()
is due to VM paging or not.
This fixes vnode recycling - where the data is forced out to swap, and
later can be paged back in, as well as paging due to memory pressure.