nfsd: Pass correct slot number to nfsd4_put_drc_mem()

In alloc_session(), numslots is the correct slot number used by the session.
But the slot number passed to nfsd4_put_drc_mem() is the one from nfs client.

Signed-off-by: Yanchuan Nian <ycnian@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>