nfsd: convert nfs4_alloc_stid() to use idr_alloc_cyclic()

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>