LU-5324 lfsck: invalidly memory access in lfsck_del_target

LU-5324 lfsck: invalidly memory access in lfsck_del_target

In lfsck_del_target(), if the given index exceeds the LFSCK targets
bitmap size, then reset the local variable @ltd to avoid to access
non-related memory unexpectedly.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ia06c24632352667cb452108ba094adea56ce5518
Reviewed-on: http://review.whamcloud.com/11060
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Jenkins
Reviewed-by: frank zago <fzago@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>