EX-5497 sec: fix encrypted symlink length

EX-5497 sec: fix encrypted symlink length

Length of encrypted symlink needs to be the length of the symlink
target. To achieve this, we need to read the symlink target and
decrypt or decode it in ->getattr(). This is possible thanks to an
enhanced getattr capable of getting more stat information.

Unfortunately, configure check for kernel support of this enhanced
getattr is incomplete, making the code call the basic getattr.

Change-Id: I03bf68679a02542b3fb696bb7458ff3276423784
Fixes: ca483b3b85 ("LU-14651 llite: extend inode methods with user namespace arg")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-on: https://review.whamcloud.com/47863
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>