LU-13717 sec: fix handling of encrypted file with long name

LU-13717 sec: fix handling of encrypted file with long name

The ciphertext representation of the name of an encrypted file or
directory can be up to 256 bytes of binary data, if the cleartext
name is up to NAME_MAX. But then this ciphertext is encoded via
critical_encode() before being sent to servers. Once encoded, the
length can exceed NAME_MAX because of the escaped critical
characters.
So make sure ll_prep_md_op_data() accepts those too long encoded names
if it is called for lookup or create of an encrypted file or
directory. In the other cases, the 'name' taken as input is the plain
text version, so it must conform to the NAME_MAX limit.

When carrying out operations on an encrypted file with long name, we
manipulate a digested form whose hash needs to be matched against the
content of the LinkEA. The name found in the LinkEA is not NUL
terminated, so this aspect must be taken care of.

Fixes: 4d38566a00 ("LU-13717 sec: filename encryption")
Fixes: ed4a625d88 ("LU-13717 sec: filename encryption - digest support")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I4b0e51eee5e549ab56292fe0fec3c1be1b487fc7
Reviewed-on: https://review.whamcloud.com/45163
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>