kconfig: move the file and lineno in struct file to struct buffer

struct file has two link nodes, 'next' and 'parent'.

The former is used to link files in the 'file_list' linked list,
which manages the list of Kconfig files seen so far.

The latter is used to link files in the 'current_file' linked list,
which manages the inclusion ("source") tree.

The latter should be tracked together with the lexer state.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>