partitions/msdos: FreeBSD UFS2 file systems are not recognized

The code in block/partitions/msdos.c recognizes FreeBSD, OpenBSD
and NetBSD partitions and does a reasonable job picking out OpenBSD
and NetBSD UFS subpartitions.

But for FreeBSD the subpartitions are always "bad".

    Kernel: <bsd:bad subpartition - ignored

Though all 3 of these BSD systems use UFS as a file system, only
FreeBSD uses relative start addresses in the subpartition
declarations.

The following patch fixes this for FreeBSD partitions and leaves
the code for OpenBSD and NetBSD intact:
Signed-off-by: Richard Narron <comet.berkeley@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/partitions/msdos.c

@@ -300,6 +300,8 @@ static void parse_bsd(struct parsed_partitions *state,
 			continue;
 		bsd_start = le32_to_cpu(p->p_offset);
 		bsd_size = le32_to_cpu(p->p_size);
+		if (memcmp(flavour, "bsd\0", 4) == 0)
+			bsd_start += offset;
 		if (offset == bsd_start && size == bsd_size)
 			/* full parent partition, we have it already */
 			continue;