staging: lustre: ptlrpc: Introduce iovec to bulk descriptor

Added a union to ptlrpc_bulk_desc for KVEC and KIOV buffers.
bd_type has been changed to be a bit mask. Bits are set in
bd_type to specify {put,get}{source,sink}{kvec,kiov}
changed all instances in the code to access the union properly
ASSUMPTION: all current code only works with KIOV and new DNE code
to be added will introduce a different code path that uses IOVEC

As part of the implementation buffer operations are added
as callbacks to be defined by users of ptlrpc.  This enables
buffer users to define how to allocate and release buffers.

Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5835
Reviewed-on: http://review.whamcloud.com/12525


Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>