LU-9480 lnet: add struct lnet_ping_buffer

LU-9480 lnet: add struct lnet_ping_buffer

The Multi-Rail code will use the ping target buffer also as the
source of data to push to other nodes. This means that there
will be multiple MDs referencing the same buffer, and care must
be taken to ensure that the buffer is not freed while any such
reference remains.

Encapsulate the struct lnet_ping_info (aka lnet_ping_info_t) in
a struct lnet_ping_buffer. This adds a reference count, and the
number of NIDs for the encapsulated lnet_ping_info has been
sized.

For sizing the buffer the constant LNET_PINGINFO_SIZE is replaced
with LNET_PING_INFO_SIZE(NNIS).

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Change-Id: Iae255a7ebd6099c050bddbea84fb1923a586ac66
Reviewed-on: https://review.whamcloud.com/25773
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Tested-by: Amir Shehata <amir.shehata@intel.com>