mm/page-writeback.c: Update comment for BANDWIDTH_INTERVAL

mm/page-writeback.c: Update comment for BANDWIDTH_INTERVAL

The name of the BANDWIDTH_INTERVAL macro is misleading, as it is not
only used in the bandwidth update functions wb_update_bandwidth() and
__wb_update_bandwidth(), but also in the dirty limit update function
domain_update_dirty_limit().

Currently, we haven't found an ideal name, so update the comment only.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Link: https://lore.kernel.org/r/20241009151728.300477-2-yizhou.tang@shopee.com
Signed-off-by: Christian Brauner <brauner@kernel.org>