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/passthrough.rs

@@ -998,11 +998,15 @@ impl FileSystem for PassthroughFs {
         offset: u64,
         _lock_owner: Option<u64>,
         _delayed_write: bool,
+        kill_priv: bool,
         _flags: u32,
     ) -> io::Result<usize> {
-        // We need to change credentials during a write so that the kernel will remove setuid or
-        // setgid bits from the file if it was written to by someone other than the owner.
-        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;
+        if kill_priv {
+            // We need to change credentials during a write so that the kernel will remove setuid
+            // or setgid bits from the file if it was written to by someone other than the owner.
+            let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;
+        }
+
         let data = self
             .handles
             .read()