LU-9679 lnet: discard lnet_ping_buffer_numref()

This inline function simply reads an atomic_t.  Having
it doesn't make the code any more readable and would
make a subsequent patch a little more awkward.
So remove it.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I21a1d2187a654f139a02c0045601086fe612e5bd
Reviewed-on: https://review.whamcloud.com/37458
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>