LU-13004 ptlrpc: remove *GET*KVEC macros and fields.

LU-13004 ptlrpc: remove *GET*KVEC macros and fields.

GET_KVEC, BD_GET_KVEC, GET_ENC_KVEC, BD_GET_ENC_KVEC
are no longer used.
There are the only users of the bd_kvec field of bd_u,
so that field can be removed, and bd_u can be discarded
and its other field (bd_kiov) promoted.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I31d19f6867c907029aa8d9ceee27c5ac9c9225a1
Reviewed-on: https://review.whamcloud.com/36972
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>