smb: client: simplify __build_path_from_dentry_optional_prefix()

Use the convenient 'strreplace()' to simplify
'__build_path_from_dentry_optional_prefix()'.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>