get_vaddr_frames() first calls pin_user_pages_fast() and if
that fails tries follow_pfn(). But if that also fails, then
the error code from pin_user_pages_fast() is overwritten with
the error code from follow_pfn().

Specifically if pin_user_pages_fast() returns -ENOMEM, then
follow_pfn() will overwrite that with -EINVAL, which is very
confusing.

So store the error code from pin_user_pages_fast() and return
that if follow_pfn() returns -EINVAL. -EINVAL indicates that
the page is unsuitable for follow_pfn, so pin_user_pages_fast()
was the correct call to make, and that error code should be
returned instead.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>