LU-8498 nodemap: new zfs index files not properly initialized

LU-8498 nodemap: new zfs index files not properly initialized

Calling index ->next on a new zfs returns a non-zero RC, but ldiskfs
indexes start with a blank record. This change modifies the config
load code to always write the default nodemap to an empty index file.

Signed-off-by: Kit Westneat <kit.westneat@gmail.com>
Change-Id: I30a365f65463979889f09f7ad5ffcdacc83fa868
Reviewed-on: http://review.whamcloud.com/21939
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>