staging: lustre: osc: Track number of "unstable" pages per osc

This change adds simple accounting hooks for "unstable" pages on a per
OSC basis. Now, in addition to the per filesystem tracking, each OSC
will maintain a running total of its unstable pages. These counters are
exported through the proc interface, and can be read using the lctl
command.

For example:

	# Read number of unstable pages contained by each OSC
	lctl get_param osc.*.unstable_stats

The motivation for this change is in anticipation of implementing a
"soft sync" functionality, urging servers to commit these unstable
pages to stable storage. The per OSC accounting allows a client to
limit the soft sync request to only the OSCs which have outstanding
unstable pages.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2139
Reviewed-on: http://review.whamcloud.com/4374


Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.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>