LU-11201 lfsck: check linkea entry validity

Invalid linkea data may lead to dead loop in linkea iteration, check
linkea entry validity on unpack, and if entry is not unpacked, check
entry length validity.

Test-Parameters: trivial mdscount=2 mdtcount=4 testlist=sanity-lfsck
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I8e1890ed64fab38b85149ebbfecce04caaf41e17
Reviewed-on: https://review.whamcloud.com/32958


Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>