media: v4l2: prepare compat-ioctl rework

The v4l2-compat-ioctl32() currently takes an extra round trip through user
space pointers when converting the data structure formats. In particular,
this involves using the compat_alloc_user_space() and copy_in_user()
helpers that often lead to worse compat handlers compared to using
in_compat_syscall() checks when copying the data.

The native implementation already gained a simpler method to deal with
the conversion for the time32 conversion.  Hook into the same places to
provide a location for reading and writing user space data from inside
of the generic video_usercopy() helper.

Hans Verkuil rewrote the video_get_user() function here to simplify
the zeroing of the extra input fields and fixed a couple of bugs in
the original implementation.

[hverkuil: fix: CHECK: Please don't use multiple blank lines]

Co-developed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>