LU-1187 fld: fix fldb proc after moving range lookup to fld.

LU-1187 fld: fix fldb proc after moving range lookup to fld.

Read fldb entries from index file directly, instead of cache, because
it might conflict with cache merging process.

Also a few fixes to make both ldiskfs and ZFS work for fldb procfs.

Change-Id: I2b8d0014b0ed0fe64c36f884f90862e720fe38e4
Signed-off-by: Wang Di <di.wang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4350
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: John Hammond <jhammond@tacc.utexas.edu>
Reviewed-by: Oleg Drokin <green@whamcloud.com>