9p: don't bother with __getname() in ->follow_link()

We copy there a kmalloc'ed string and proceed to kfree that string immediately
after that.  Easier to just feed that string to nd_set_link() and _not_
kfree it until ->put_link() (which becomes kfree_put_link() in that case).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>