dm mpath: replace spin_lock_irqsave with spin_lock_irq

Replace spin_lock_irqsave/spin_unlock_irqrestore with
spin_lock_irq/spin_unlock_irq at places where it is known that interrupts
are enabled.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>