LU-4423 ptlrpc: make ptlrpc_bulk_frag_ops always const.

LU-4423 ptlrpc: make ptlrpc_bulk_frag_ops always const.

There is one place where a non-const pointer to this struct
exists, and that causes a cast to be required.

Make it always const, and discard the cast.

Test-Parameters: trivial

Change-Id: Ic48ff6a1f759abdddc75298feee2b68c60bf3e7b
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35295
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>