xen/blkback: don't fail empty barrier requests

xen/blkback: don't fail empty barrier requests

The sector number on empty barrier requests may (will?) be
uninitialized (neither bio_init() nor rq_init() set the respective
fields), which allows for exceeding the actual (virtual) disk's size.

Inspired by Konrad's "When writting barriers set the sector number to
zero...", but instead of zapping the sector number (which is wrong for
non-empty ones) just ignore the sector number when the sector count is
zero.

While at it also add overflow checking to the math in vbd_translate().

Signed-off-by: Jan Beulich <jbeulich@novell.com>