- Use fd_lastfile + 1 as the upper bound on nd. This is more correct than

 - Use fd_lastfile + 1 as the upper bound on nd.  This is more correct than
   using the size of the descriptor array.
 - A lock is not needed to fetch fd_lastfile.  The results are stale the
   instant it is dropped.
 - Use a private mutex pool for select since the pool mutex is not used
   as a leaf.
 - Fetch the si_mtx pointer first before resorting to hashing to compute
   the mutex address.

Reviewed by:	McKusick
Approved by:	re (kib)