[vboxsf] Fix reading files when more than 2 gigs of memory available

When the VM was configured with over 2 gigabytes of memory, files read
from shared folders contained corrupted data.

Thanks to Richard Braakman for the original patch.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
Merge pull request #1 from kaltsi/highmem-fix

[vboxsf] Fix reading files when more than 2 gigs of memory available