fuse: add request extension

[ Upstream commit 15d937d7 ]

Will need to add supplementary groups to create messages, so add the
general concept of a request extension.  A request extension is appended to
the end of the main request.  It has a header indicating the size and type
of the extension.

The create security context (fuse_secctx_*) is similar to the generic
request extension, so include that as well in a backward compatible manner.

Add the total extension length to the request header.  The offset of the
extension block within the request can be calculated by:

  inh->len - inh->total_extlen * 8

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Stable-dep-of: 3002240d ("fuse: fix memory leak in fuse_create_open")
Signed-off-by: Sasha Levin <sashal@kernel.org>