John Sperbeck reported that if we turn off CONFIG_RFS_ACCEL, the 'head'
is not defined, which will trigger compile error. So I move the 'head'
out of the CONFIG_RFS_ACCEL scope.

Fixes: 84b6823c ("net: rps: protect last_qtail with rps_input_queue_tail_save() helper")
Reported-by: John Sperbeck <jsperbeck@google.com>
Closes: https://lore.kernel.org/all/20240529203421.2432481-1-jsperbeck@google.com/Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240530032717.57787-1-kerneljasonxing@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>