buffer: remove useless comment and WB_REASON_FREE_MORE_MEM, reason.

free_more_memory func has been completely removed in commit bc48f001
("buffer: eliminate the need to call free_more_memory() in __getblk_slow()")

So comment and `WB_REASON_FREE_MORE_MEM` reason about free_more_memory
are no longer needed.

Fixes: bc48f001 ("buffer: eliminate the need to call free_more_memory() in __getblk_slow()")
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/buffer.c

@@ -967,7 +967,7 @@ grow_dev_page(struct block_device *bdev, sector_t block,
 	struct page *page;
 	struct buffer_head *bh;
 	sector_t end_block;
-	int ret = 0;		/* Will call free_more_memory() */
+	int ret = 0;
 	gfp_t gfp_mask;
 
 	gfp_mask = mapping_gfp_constraint(inode->i_mapping, ~__GFP_FS) | gfp;

include/linux/backing-dev-defs.h

@@ -54,7 +54,6 @@ enum wb_reason {
 	WB_REASON_SYNC,
 	WB_REASON_PERIODIC,
 	WB_REASON_LAPTOP_TIMER,
-	WB_REASON_FREE_MORE_MEM,
 	WB_REASON_FS_FREE_SPACE,
 	/*
 	 * There is no bdi forker thread any more and works are done

include/trace/events/writeback.h

@@ -36,7 +36,6 @@
 	EM( WB_REASON_SYNC,			"sync")			\
 	EM( WB_REASON_PERIODIC,			"periodic")		\
 	EM( WB_REASON_LAPTOP_TIMER,		"laptop_timer")		\
-	EM( WB_REASON_FREE_MORE_MEM,		"free_more_memory")	\
 	EM( WB_REASON_FS_FREE_SPACE,		"fs_free_space")	\
 	EMe(WB_REASON_FORKER_THREAD,		"forker_thread")