benchmarks/blk_mt: fix error messages

Ref: pmem/issues#11
These changes look good and normally I wouldn't worry about this case too much, but since you are in here fixing things already... :-)

It seems like pthread_create() and pthread_join() return an error number that you throw away and print "failed" instead.  You could change those two calls to use our usual idiom:

`if ((errno = pthread_create(...)) !=  0)`

and then use warn() to print the error message.