fuse: introduce the notion of FUSE_HANDLE_KILLPRIV_V2

We already have FUSE_HANDLE_KILLPRIV flag that says that file server will
remove suid/sgid/caps on truncate/chown/write. But that's little different
from what Linux VFS implements.

To be consistent with Linux VFS behavior what we want is.

- caps are always cleared on chown/write/truncate
- suid is always cleared on chown, while for truncate/write it is cleared
  only if caller does not have CAP_FSETID.
- sgid is always cleared on chown, while for truncate/write it is cleared
  only if caller does not have CAP_FSETID as well as file has group execute
  permission.

As previous flag did not provide above semantics. Implement a V2 of the
protocol with above said constraints.

Server does not know if caller has CAP_FSETID or not. So for the case
of write()/truncate(), client will send information in special flag to
indicate whether to kill priviliges or not. These changes are in subsequent
patches.

FUSE_HANDLE_KILLPRIV_V2 relies on WRITE being sent to server to clear
suid/sgid/security.capability. But with ->writeback_cache, WRITES are
cached in guest. So it is not recommended to use FUSE_HANDLE_KILLPRIV_V2
and writeback_cache together. Though it probably might be good enough
for lot of use cases.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>