af_key: Add lock to key dump

commit 89e357d83c06b6fac581c3ca7f0ee3ae7e67109e upstream.

A dump may come in the middle of another dump, modifying its dump
structure members. This race condition will result in NULL pointer
dereference in kernel. So add a lock to prevent that race.

Fixes: 83321d6b ("[AF_KEY]: Dump SA/SP entries non-atomically")
Signed-off-by: Yuejie Shi <syjcnss@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
[bwh: Backported to 3.2:
 - pfkey_dump() doesn't support filters
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>