LU-13004 ptlrpc: simplify bd_vec access.

LU-13004 ptlrpc: simplify bd_vec access.

Now that there are no kvecs in ptlrpc_bulk_desc, only bdvecs, we can
simplify the access, discarding the containing struct and the macros,
and just accessing the fields directly.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I068a7a280f130bf0b53b9c572ed47ef0cc999102
Reviewed-on: https://review.whamcloud.com/36973
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>