LU-9558 llite: generic_readlink is no longer exported

Starting in linux kernel 4.10 if inode_operation's
readlink field is NULL then it is assumed to be using
generic_readlink. Since this is the case the function
generic_readlink is no longer visible to individual
file systems.

Linux-commit: dfeef68862edd7d4bafe68ef7aeb5f658ef24bb5

Change-Id: I847844cc53f7ae80d9d212590f5026a92da0107e
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/27280
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>