NFS: Cleanup error handling in nfs_idmap_request_key()

nfs_idmap_get_desc() can't actually return zero.  But if it did then
we would return ERR_PTR(0) which is NULL and the caller,
nfs_idmap_get_key(), doesn't expect that so it leads to a NULL pointer
dereference.

I've cleaned this up by changing the "<=" to "<" so it's more clear that
we don't return ERR_PTR(0).

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>