sysfs: Allow symlinks to be added between sibling groups

A subsequent commit has the need to create a symlink from an attribute
in a first group to an attribute in a second group.  Both groups belong
to the same kobject.

More specifically, each signature received from an authentication-
capable device is going to be represented by a file in the first group
and shall be accompanied by a symlink pointing to the certificate slot
in the second group which was used to generate the signature (a device
may have multiple certificate slots and each is represented by a
separate file in the second group):

/sys/devices/.../signatures/0_certificate_chain -> .../certificates/slot0

There is already a sysfs_add_link_to_group() helper to add a symlink to
a group which points to another kobject, but this isn't what's needed
here.

So add a new function to add a symlink among sibling groups of the same
kobject.

The existing sysfs_add_link_to_group() helper goes through a locking
dance of acquiring sysfs_symlink_target_lock in order to acquire a
reference on the target kobject.  That's unnecessary for the present
use case as the link itself and its target reside below the same
kobject.

To simplify error handling in the newly introduced function, add a
DEFINE_FREE() clause for kernfs_put().

Signed-off-by: Lukas Wunner <lukas@wunner.de>