LU-11201 lfsck: check linkea entry validity

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: a5441a717c3a97071494ff51cfb72a117d12d96e

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>