scsi: pvscsi: limit loop to fetch SG list

In PVSCSI paravirtual SCSI bus, pvscsi_convert_sglist can take a very
long time or go into an infinite loop due to two different bugs:

1) the request descriptor data length is defined to be 64 bit. While
building SG list from a request descriptor, it gets truncated to 32bit
in routine 'pvscsi_convert_sglist'. This could lead to an infinite loop
situation large 'dataLen' values when data_length is cast to uint32_t and
chunk_size becomes always zero.  Fix this by removing the incorrect cast.

2) pvscsi_get_next_sg_elem can be called arbitrarily many times if the
element has a zero length.  Get out of the loop early when this happens,
by introducing an upper limit on the number of SG list elements.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-Id: <1473108643-12983-1-git-send-email-ppandit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
