LU-2511 procfs: Disallow secretly (un)writable proc entries

LU-2511 procfs: Disallow secretly (un)writable proc entries

Add an assert to lprocfs_seq_create() to ensure that proc entries
implementing write have user writable modes.  Fix a few callers to
pass this assertion.

Signed-off-by: John L. Hammond <jhammond@tacc.utexas.edu>
Change-Id: I95d08106f7d9be478c29d7331cdf5ab607435889
Reviewed-on: http://review.whamcloud.com/4871
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>