nfsd: increase DRC cache limit

nfsd: increase DRC cache limit

An NFSv4.1+ client negotiates the size of its duplicate reply cache size
in the initial CREATE_SESSION request.  The server preallocates the
memory for the duplicate reply cache to ensure that we'll never fail to
record the response to a nonidempotent operation.

To prevent a few CREATE_SESSIONs from consuming all of memory we set an
upper limit based on nr_free_buffer_pages().  1/2^10 has been too
limiting in practice; 1/2^7 is still less than one percent.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
(cherry picked from commit 44d8660d3bb0a1c8363ebcb906af2343ea8e15f6)

Orabug: 28023821
Signed-off-by: Manjunath Patil <manjunath.b.patil@oracle.com>
Reviewed-by: Calum Mackay <calum.mackay@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>