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

Lustre-change: https://review.whamcloud.com/32958


Lustre-commit: a5441a71

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I8e1890ed64fab38b85149ebbfecce04caaf41e17
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33078


Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>