LU-16670 enc: make sure DoM files are correctly decrypted

LU-16670 enc: make sure DoM files are correctly decrypted

Make sure DoM files are decrypted upon read by loading their
associated encryption context, via llcrypt_prepare_readdir()/
llcrypt_get_encryption_info().

Fix sanity-sec test_50 accordingly.

Lustre-change: https://review.whamcloud.com/50429
Lustre-commit: 1c424252d37c64e3c223c19dced3cad2649c1f61

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ie9ef3cbb08d2295a2fd10b9e9ab0862119c7723e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50430
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>