sunrpc: add SUNRPC_CMD_CACHE_FLUSH netlink command

sunrpc: add SUNRPC_CMD_CACHE_FLUSH netlink command

Add a new SUNRPC_CMD_CACHE_FLUSH generic netlink command that allows
userspace to flush the sunrpc auth caches (ip_map and unix_gid) without
writing to /proc/net/rpc/*/flush.

An optional SUNRPC_A_CACHE_FLUSH_MASK u32 attribute selects which caches
to flush (bit 1 = ip_map, bit 2 = unix_gid). If the attribute is
omitted, all sunrpc caches are flushed.

This is used by exportfs to replace its /proc-based cache_flush() with a
netlink equivalent, with /proc fallback for older kernels.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>