cifs: rename reconn_inval_dfs_target()

This function has nothing to do with *invalidation* but setting up the
next target server from a cached referral.

Rename it to reconn_set_next_dfs_target().  While at it, get rid of
some meaningless checks.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>