vdso: x86 -- Drop BUG_ON if no pfn found

If pfn = 0 it means we hit something very strange
condition but better to not yield BUG_ON here,
better exit with error for future investigation.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>