staging: exfat: Use kmemdup in exfat_symlink()

Use kmemdup rather than duplicating its implementation

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20190905030047.88401-1-yuehaibing@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>