ksmbd: handle caseless file creation

Ray Zhang reported ksmbd can not create file if parent filename is
caseless.

Y:\>mkdir A
Y:\>echo 123 >a\b.txt
The system cannot find the path specified.
Y:\>echo 123 >A\b.txt

This patch convert name obtained by caseless lookup to parent name.

Cc: stable@vger.kernel.org # v5.15+
Reported-by: Ray Zhang <zhanglei002@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>