nfsd: fix performance-limiting session calculation

We're unintentionally limiting the number of slots per nfsv4.1 session
to 10.  Often more than 10 simultaneous RPCs are needed for the best
performance.

This calculation was meant to prevent any one client from using up more
than a third of the limit we set for total memory use across all clients
and sessions.  Instead, it's limiting the client to a third of the
maximum for a single session.

Fix this.

Reported-by: Chris Tracy <ctracy@engr.scu.edu>
Cc: stable@vger.kernel.org
Fixes: de766e57

 "nfsd: give out fewer session slots as limit approaches"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>