samples/vfs: check whether flag was raised

For string options the kernel will raise the corresponding flag only if
the string isn't empty. So check for the flag.

Link: https://lore.kernel.org/r/20250204-work-mnt_idmap-statmount-v2-3-007720f39f2e@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>