knfsd: fix reply cache memory corruption

Fix a regression in the reply cache introduced when the code was
converted to use proper Linux lists.  When a new entry needs to be
inserted, the case where all the entries are currently being used
by threads is not correctly detected.  This can result in memory
corruption and a crash.  In the current code this is an extremely
unlikely corner case; it would require the machine to have 1024
nfsd threads and all of them to be busy at the same time.  However,
upcoming reply cache changes make this more likely; a crash due to
this problem was actually observed in field.

Signed-off-by: Greg Banks <gnb@sgi.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>