[SCSI] sg direct io/mmap oops, st sync

This patch adopts the same solution as proposed by Kai M. in
a post titled: "[PATCH] SCSI tape signed/unsigned fix".
The fix is in a function that the sg driver borrowed from
the st driver so its maintenance is a little easier if
the functions remain the same after the fix.

   - change nr_pages type from unsigned to signed so errors
     from get_user_pages() call are properly handled

Signed-off-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>