LU-16374 enc: rename O_FILE_ENC to O_CIPHERTEXT

LU-16374 enc: rename O_FILE_ENC to O_CIPHERTEXT

Rename O_FILE_ENC to O_CIPHERTEXT as per discussion in linux-fscrypt
mailing-list.
Also change the flag combination to be:
O_NOCTTY | O_NDELAY | O_DSYNC
to avoid the risk of accidental issues with tar that already opens
files with the 'O_NOCTTY | O_NDELAY' combination.

O_DSYNC does not make much sense for O_RDONLY files, but will force
writes on encrypted restore to be synchronous. With O_DIRECT and large
enough writes (32MB?) that might be OK, but not ideal for small files.

Fixes: fdbf2ffd41 ("LU-14677 sec: no encryption key migrate/extend/resync/split")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I36fed17a413ee690bc445c3e76674ed5fc337de5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51640
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>