drm/i915: Use pagecache write to prepopulate shmemfs from pwrite-ioctl

Before we instantiate/pin the backing store for our use, we
can prepopulate the shmemfs filp efficiently using a write into the
pagecache. We avoid the penalty of instantiating all the pages, important
if the user is just writing to a few and never uses the object on the GPU,
and using a direct write into shmemfs allows it to avoid the cost of
retrieving a page (mostly the clear-before-use, but in theory we could
curtail swapin) before it is overwritten.

This can be extended later to provide additional specialisation for
other backends (other than shmemfs). For now it provides a defense
against very large write-only allocations from exhausting all of system
memory.

v2: Smelling fixes.

Fixes: fe115628d567 ("drm/i915: Implement pwrite without struct-mutex")
References: https://bugs.freedesktop.org/show_bug.cgi?id=99107
Signed-off-by: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: <[email protected]> # v4.10+
Reviewed-by: Joonas Lahtinen <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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