ipv4: convert dst_metrics.refcnt from atomic_t to refcount_t

ipv4: convert dst_metrics.refcnt from atomic_t to refcount_t

BugLink: http://bugs.launchpad.net/bugs/1746474
refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 9620fef27ed2cdb37bf6fd028f32bea2ef5119a8)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>