LU-12602 mdt: check EA size in mdt_getxattr_pack_reply()

LU-12602 mdt: check EA size in mdt_getxattr_pack_reply()

Check EA data size(non-positive or excessively large) in case of
any corruption.

Change-Id: I8ccea214f8d7c0403a9df180acf487ee381b8d77
Reported-by: Alibaba Cloud <yunye.ry@alibaba-inc.com>
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35768
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>