nfsd: don't alloc under spinlock in rpc_parse_scope_id

Dan Carpenter says:

  The patch d20c11d8: "nfsd: Protect session creation and client
  confirm using client_lock" from Jul 30, 2014, leads to the following
  Smatch static checker warning:

        net/sunrpc/addr.c:178 rpc_parse_scope_id()
        warn: sleeping in atomic context

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: d20c11d8 ("nfsd: Protect session creation and client...")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>