dbuf_cons: deduplicate multilist_link_init()

Remove harmless duplicate multilist_link_init() introduced by
commit d3c2ae1.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Closes #6552