LU-2363 lod: Fix statfs entries in lod procfs

LU-2363 lod: Fix statfs entries in lod procfs

Factor toplevel lod procfs out of lod_pools_{init,fini}.  Support
lod statfs entries using osd methods.

Change-Id: I27f8c1441079ad455a03f16abb05491fd655d2e3
Signed-off-by: John L. Hammond <jhammond@tacc.utexas.edu>
Reviewed-on: http://review.whamcloud.com/4637
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <kemannthey@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>