Fix the default number of threads for Flex File layout pNFS client I/O.

Fix the default number of threads for Flex File layout pNFS client I/O.

The intent was that the default would be based on number of CPUs, but the
code disabled using taskqueue() by default.
This code is only executed when mounting a NFSv4.1 server that supports the
Flexible File layout for pNFS and, since such servers are rare, this change
shouldn't result in a POLA violation.
(The FreeBSD pNFS server is still a project and the only other one that
 uses Flexible File layout is being developed by Primary Data and I don't
 know if they have even shipped any to customers yet.)
Found while testing the pNFS server.