ubifs: Drop softlimit and delta fields from struct ubifs_wbuf

Values of these fields are set during init and never modified. They are
used (read) in a single function only. There isn't really any reason to
keep them in a struct. It only makes struct just a bit bigger without
any visible gain.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>