tc: m_gate: use SPRINT_BUF when needed

tc: m_gate: use SPRINT_BUF when needed

sprint_time64() uses SPRINT_BSIZE-1 as a constant buffer lenght in its
implementation, however m_gate uses shorter buffers when calling it.

Fix this using SPRINT_BUF macro to get the buffer, thus getting a
SPRINT_BSIZE-long buffer.

Fixes: 07d5ee70b5b3 ("iproute2-next:tc:action: add a gate control action")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>