xsk: Fix refcount warning in xp_dma_map

Fix a potential refcount warning that a zero value is increased to one
in xp_dma_map, by initializing the refcount to one to start with,
instead of zero plus a refcount_inc().

Fixes: 921b6869

 ("xsk: Enable sharing of dma mappings")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/1600095036-23868-1-git-send-email-magnus.karlsson@gmail.com