net: bridge: don't increment tx_dropped in br_do_proxy_arp

pskb_may_pull may fail due to various reasons (e.g. alloc failure), but the
skb isn't changed/dropped and processing continues so we shouldn't
increment tx_dropped.

CC: Kyeyoon Park <kyeyoonp@codeaurora.org>
CC: Roopa Prabhu <roopa@cumulusnetworks.com>
CC: Stephen Hemminger <stephen@networkplumber.org>
CC: bridge@lists.linux-foundation.org
Fixes: 95850116 ("bridge: Add support for IEEE 802.11 Proxy ARP")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>