vfs: Call parent directory dentry 'newdirdp' in sys_link()

Call the dentry that refers to the newpath parent directory "newdirdp"
to make the code more explicit.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs: Simplify sys_link() unwinding

We can safely use the same function exit points for both successful and
error cases.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>