vfs: Fix hard link on subsequent calls for the same dentry

The link system call currently keeps the dentry in the cache by avoiding
calling drele on success.  However, it's not the right thing as link
creates the entry, not references it (much the same as create).
Currently, link works ok, but subsequent calls on the same dentry would
fail as its refcount would be wrong.

Strangely, this problem didn't come up at the time the link syscall was
implemented.  Something that has been changed recently made this bug
manifest.

This patch fixes it.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Add tst-fs-link.so to the whitelist of test.py

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>