smb/client: ensure smb2_mapping_table rebuild on cmd changes

smb/client: ensure smb2_mapping_table rebuild on cmd changes

The current rule for smb2_mapping_table.c uses `$(call cmd,...)`, which
fails to track command line modifications in the Makefile (e.g., modifying
the command to `perl -d` or `perl -w` for debug will not trigger a rebuild)
and does not generate the required .cmd file for Kbuild.

Fix this by transitioning to the standard `$(call if_changed,...)` macro.
This includes adding the `FORCE` prerequisite and appending the output
file to the `targets` variable so Kbuild can track it properly.

As a result, Kbuild now automatically handles the cleaning of the
generated file, allowing us to safely drop the redundant `clean-files`
assignment.

Fixes: c527e13a7a66 ("cifs: Autogenerate SMB2 error mapping table")
Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>