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

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

Lustre uses ZAP to implement index object. When tar the index
object via backend ZPL for backup, it is explained as regular
file, then when untar it, it is not ZAP formatted again, then
the Lustre cannot recognize the 'bad' formatted index object.

On the other hand, each backend FS has its own special format
for index object. Then we cannot migrate the index files from
one backend to another directly.

To resolve such issue, the patch will backup the index object
with plain format to the local '/index_backup' directory with
the name of source index's FID string and ".lbx" postfix when
umount the device.

The format of the backup is as following:
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.

Test-Parameters: envdefinitions=SLOW=yes testlist=sanity-scrub mdtfilesystemtype=zfs ostfilesystemtype=zfs mdscount=2 mdtcount=4
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I01730bc9cfa3ae597f2d8652df9fb76418cf55ce
Reviewed-on: https://review.whamcloud.com/30910
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>