It would be possible to put any statement in TP_fast_assign().

This commit obsoletes the helper function and put its statements to
TP_fast_assign() for the code simplicity.

Link: https://lore.kernel.org/r/20240712003010.87341-1-o-takashi@sakamocchi.jpSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>