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Always return an ERR_PTR() instead of NULL.

This makes the behavior of alloc, get_userptr and attach_dmabuf the
same.

Update the documentation in videobuf2-core.h as well.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>