LU-2467 ptlrpc: Allow OBD_PINGs to be suppressed

This patch introduces a new ptlrpc module parameter, "suppress_pings",
to provide an option for reducing excessive OBD_PINGs in large
clusters.  The parameter affects all MDTs and OSTs on a node.  It is
off (zero) by default, giving a behavior identical to current
implementation.  If it is on (non-zero), all clients of the affected
targets who understand OBD_CONNECT_PINGLESS will know, at connect
time, that pings are not required.  When suppressing pings, there must
be an external mechanism to notify the targets of client deaths, via
the targets' "evict_client" procfs entries.  In addition, a highly
available standalone MGS is also recommended when suppressing pings,
so that clients are notified (through Imperative Recovery) of target
recoveries.

The changes do two basically independent things.  One is initializing
import and export states (i.e., imp_connect_data and
exp_obd_chain_timed) according to "suppress_pings", since whether to
ping or not is a property of each import-export pair.  MGC pings can
not be suppressed, because maintaining MGS connections is dictated by
the reliance on Imperative Recovery.

The other thing is changing pinger and import routines to respect the
import property set earlier.  (The export side does not need any
change at all.)  Pings are still needed to query last committed
transactions if there are uncommitted requests on an import, so that
resources pinned for replays can be released even when applications
become idle.  An early version of this patch removes imports that do
not need to be pinged from pinger_imports and add them back when last
committed transactions are needed or recoveries must be initiated.
This version does not do that because a) the overheads of iterating
through 10,000 imports are not prohibitively large---around 10 ms, b)
adding imports back to pinger_imports requires the global pinger_mutex
to be held, and c) the imp_lock contention added on each import is
small.

Change-Id: Iabc84d395c978c3f156c52aebfad83621facb4fe
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/5009
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Hudson
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>