Avoid the nbp lookup in the final loop iteration in flushbuflist().

The end of the loop must re-lookup the next buf since the bufobj lock
is dropped in the loop body. If the lookup fails, the loop is restarted.
This mechanism non-obviously also terminates the loop when the end of
the buf list is reached. Split up the two loops termination cases to
make the code a bit less fragile. No functional change intended.

Reviewed by: kib
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12730