linux: libshare: smb: fix more than one smb_is_share_active() call

This also fixes zfs_unshare_006_pos, which exposed this

Fixes: 2f71caf2d926249920d1b9162550c56715cc6461 ("Allow zfs unshare
<protocol> -a")

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13259