LU-12604 mdt: check field size of sec context name

LU-12604 mdt: check field size of sec context name

In request received from client, check that claimed size of
RMF_FILE_SECCTX_NAME field is consistent with expected content,
which is supposed to be an extended attribute name.

Test-Parameters: clientselinux testlist=sanity,recovery-small,sanity-selinux envdefinitions=SANITY_EXCEPT="271f"
Reported-by: Alibaba Cloud <yunye.ry@alibaba-inc.com>
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ice96f0e03f790b334fcdf64ae4becef2e39738f4
Reviewed-on: https://review.whamcloud.com/35655
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>