bio: add bvec_length(), like iov_length()

Change-Id: Idfc33569ab06377514176a4a75bd281781a30eff
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: Zach Brown <zab@zabbo.net>
Signed-off-by: zwliew <zhaoweiliew@gmail.com>