Expand vaddr_to_pfn() so it can read and translate to PFN
from already opened fd. This can be used as optimization
if one need to translate a couple of addresses and
also to read other task's PFN.

I'll use it in the next patch for reading other tasks's
vdso's PFN.
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>