LU-6391 llite: Add client mount opt to ignore suppress_pings

LU-6391 llite: Add client mount opt to ignore suppress_pings

When Lustre servers enable 'suppress_pings', all clients will stop
pinging. However, some clients may not have external mechanism
to notify Lustre servers for node death and therefore need to
preserve the Lustre ping.

This patch provides a mount option 'always_ping' so that the
client will not stop pinging even if the server has enabled
'suppress_pings'.

Signed-off-by: Wally Wang <wang@cray.com>
Change-Id: Ia7b45e8d2dbb53f02157ef2ab1d327d9483c2455
Reviewed-on: http://review.whamcloud.com/14127
Tested-by: Jenkins
Reviewed-by: Li Wei <wei.g.li@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>