Currently during vdev delete host will flush threads in serial
order, which can take upto 20 seconds for 5 threads in defconfig
build (Per thread wait timeout is 4 seconds in defconfig build),
this is turn can result in timeout in caller function, which has
maximum wait timeout of 10 seconds.
To optimize first flush all the threads nbuf list, after
flushing all the thread nbuf list wait for event completion
on threads.
CRs-Fixed: 3673505
Change-Id: I0c2979e7b08f27872da048c4a709800fc46eb526
51d026afd1