multipathd: fixup locking in uev_remove_map()

We need to take the vector lock early in uev_remove_map() to
avoid segfault during reconfiguration.

References: bnc#901891

Signed-off-by: ShivaKrishna Merla <shivakrisna.merla@netapp.com>
Acked-by: Hannes Reinecke <hare@suse.de>