kconfig: change file_lookup() to return the file name

kconfig: change file_lookup() to return the file name

Currently, file_lookup() returns a pointer to (struct file), but the
callers use only file->name.

Make it return the ->name member directly.

This adjustment encapsulates struct file and file_list as internal
implementation.

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