cifsd: get parent dentry from child in ksmbd_vfs_remove_file()

To remove the file, We have parsed full pathname to divide parent path and
filename. It is a better way to get parent dentry from child dentry that
obtained by lookup with given pathname.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>