Add Frontswap
Frontswap is so named because it can be thought of as the opposite of
a 'backing' store for a swap device.  The storage is assumed to be
a synchronous concurrency-safe page-oriented pseudo-RAM device (such as
Xen's Transcendent Memory, aka 'tmem', or in-kernel compressed memory,
aka 'zmem', or other RAM-like devices) which is not directly accessible
or addressable by the kernel and is of unknown and possibly time-varying
size.  This pseudo-RAM device links itself to frontswap by setting the
frontswap_ops pointer appropriately and the functions it provides must
conform to certain policies as follows:

An 'init' prepares the pseudo-RAM to receive frontswap pages and returns
a non-negative pool id, used for all swap device numbers (aka 'type').
A 'put_page' will copy the page to pseudo-RAM and associate it with
the type and offset associated with the page. A 'get_page' will copy the
page, if found, from pseudo-RAM into kernel memory, but will NOT remove
the page from pseudo-RAM.  A 'flush_page' will remove the page from
pseudo-RAM and a 'flush_area' will remove ALL pages associated with the
swap type (e.g., like swapoff) and notify the pseudo-RAM device to refuse
further puts with that swap type.

Once a page is successfully put, a matching get on the page will always
succeed.  So when the kernel finds itself in a situation where it needs
to swap out a page, it first attempts to use frontswap.  If the put returns
non-zero, the data has been successfully saved to pseudo-RAM and
a disk write and, if the data is later read back, a disk read are avoided.
If a put returns zero, pseudo-RAM has rejected the data, and the page can
be written to swap as usual.

Note that if a page is put and the page already exists in pseudo-RAM
(a 'duplicate' put), either the put succeeds and the data is overwritten,
or the put fails AND the page is flushed.  This ensures stale data may
never be obtained from pseudo-RAM.
Overclock from old kernel