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[Why]
Userspace should get back a copy of drm_wait_vblank that's been modified
even when drm_wait_vblank_ioctl returns a failure.

Rationale:
drm_wait_vblank_ioctl modifies the request and expects the user to read
it back. When the type is RELATIVE, it modifies it to ABSOLUTE and updates
the sequence to become current_vblank_count + sequence (which was
RELATIVE), but now it became ABSOLUTE.
drmWaitVBlank (in libdrm) expects this to be the case as it modifies
the request to be Absolute so it expects the sequence to would have been
updated.

The change is in compat_drm_wait_vblank, which is called by
drm_compat_ioctl. This change of copying the data back regardless of the
return number makes it en par with drm_ioctl, which always copies the
data before returning.

[How]
Return from the function after everything has been copied to user.

Fixes IGT:kms_flip::modeset-vs-vblank-race-interruptible
Tested on ChromeOS Trogdor(msm)
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Mark Yacoub <markyacoub@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210812194917.1703356-1-markyacoub@chromium.org