fs/9p: TREADLINK bugfix

Remove v9fs_vfs_readlink_dotl function and use generic_readlink. Update
v9fs_vfs_follow_link_dotl function to accommodate this change

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Reported-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>