drm/i915: Report -EFAULT before pwrite fast path into shmemfs

When pwriting into shmemfs, the fast path pagecache_write does not
notice when it is writing to beyond the end of the truncated shmemfs
inode. Report -EFAULT directly when we try to use pwrite into the
!I915_MADV_WILLNEED object.

Fixes: 7c55e2c5

 ("drm/i915: Use pagecache write to prepopulate shmemfs from pwrite-ioctl")
Testcase: igt/gem_madvise/dontneed-before-pwrite
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171016202732.25459-1-chris@chris-wilson.co.uk