[multipathd] fix uevent trigger fn return values
[multipathd] fix uevent trigger fn return values

o uevent from a non-/block object is discarded without error
o unknown dm- object uevents are discarded without error
o lock a bit earlier for unlock symmetry in the out path

The bogus and confusing "uevent trigger error" log messages 
are now gone.


unakatsuo/multipath-tools,vijaychauhan/multipath-tools,vijaychauhan/multipath-tools,unakatsuo/multipath-tools,unakatsuo/multipath-tools,grzn/multipath-tools-explained,gebi/multipath-tools,grzn/multipath-tools-explained