LU-15810 sec: fix enc dir migration

LU-15810 sec: fix enc dir migration

Now that the encryption context is stored in an xattr named
"encryption.c" instead of "security.c", we need to fetch this xattr
explicitly in case of encrypted directory migration. Indeed, there is
no xattr handler in ldiskfs for this "encryption." xattr type, so it
is not returned when listing all xattrs to migrate.

Fixes: 4231fab66e ("LU-13717 sec: make client encryption compatible with ext4")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I628f9b253e86343db0b71f6a5b1ad2c5728ca38d