LU-10193 osd-ldiskfs: backup index object with plain format

This patch is mainly for migrating filesyste between ZFS
backend and ldiskfs backend via server file level backup
and restore. It will dumps the ldiskfs special formatted
index object to the local '/index_backup' directory with
the name of source index's FID string and ".lbx" postfix
when umount device.

The format of the backup is as following (same ZFS case):
1) header: 512 bytes, including:
   magic:       4 bytes
   count:       4 bytes
   keysize:     4 bytes
   recsize:     4 bytes
   owner_fid:   16 bytes
   padding:     480 bytes

2) body: after the header, <key, rec> pairs one by one.

The backup will be done when server umount. The backup behavior
is controlled via new OSD lproc interface "index_backup". It is
off by default. You can turn it on to enable backup when server
umount via writing non-zero value to such lproc interface.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I5ac81dd470f3cb29eb3c9ec0e01935c9b1a0fda9
Reviewed-on: https://review.whamcloud.com/30911
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>