smb: client: fix conflicting option validation for new mount API

smb: client: fix conflicting option validation for new mount API

Apply conflicting option validation consistently across all the new
mount API paths, for both mount and remount.

Some checks were only applied during initial mount validation, while
others were handled during option parsing, causing mount and
remount/reconfigure to behave differently.

Move the conflicting option checks into smb3_handle_conflicting_options()
and call it from the common validation paths, including for
multichannel/max_channels handling.

Fixes: 24e0a1eff9e2 ("cifs: switch to new mount api")
Signed-off-by: Henrique Carvalho <henrique.carvalho@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>