nfsd: count nfsd_file allocations

We already count the frees (via nfsd_file_releases). Count the
allocations as well. Also switch the direct call to nfsd_file_slab_free
in nfsd_file_do_acquire to nfsd_file_free, so that the allocs and
releases match up.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>