vhost_user_fs: Add the WRITE_KILL_PRIV write flag.

Add the WRITE_KILL_PRIV write flag, corresponding to
FUSE_WRITE_KILL_PRIV introduced in 7.31, and use to only remove the
setuid and setgid bits (by switching credentials) conditionally.

Signed-off-by: Sergio Lopez <slp@redhat.com>

vhost_user_fs/src/fuse.rs

@@ -373,6 +373,9 @@ pub const WRITE_CACHE: u32 = 1;
 /// `lock_owner` field is valid.
 pub const WRITE_LOCKOWNER: u32 = 2;
 
+/// Kill suid and sgid bits
+pub const WRITE_KILL_PRIV: u32 = 4;
+
 // Read flags.
 pub const READ_LOCKOWNER: u32 = 2;