LU-4941 lfsck: check LOV EA header properly

LU-4941 lfsck: check LOV EA header properly

1) If the LFSCK does not recognize the magic, then check whether
   the magic's postfix match the LOV_MAGIC_MAGIC or not: if yes,
   then it is quite possible that the system was downgraded from
   a new release with new LOV magic, so it is better to keep the
   current LOV EA unchanged; otherwise, it is crashed LOV EA and
   should be repaired by the LFSCK.

2) Check the pattern with lov_pattern().

3) Drop redundant LOV EA header verification.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iad3a1d6fc50a5dfba977a534fdb253bc9f6373e5
Reviewed-on: http://review.whamcloud.com/10045
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>