fs: ratelimit __find_get_block_slow() failure message.

When something let __find_get_block_slow() hit all_mapped path, it calls
printk() for 100+ times per a second. But there is no need to print same
message with such high frequency; it is just asking for stall warning, or
at least bloating log files.

  [  399.866302][T15342] __find_get_block_slow() failed. block=1, b_blocknr=8
  [  399.873324][T15342] b_state=0x00000029, b_size=512
  [  399.878403][T15342] device loop0 blocksize: 4096
  [  399.883296][T15342] __find_get_block_slow() failed. block=1, b_blocknr=8
  [  399.890400][T15342] b_state=0x00000029, b_size=512
  [  399.895595][T15342] device loop0 blocksize: 4096
  [  399.900556][T15342] __find_get_block_slow() failed. block=1, b_blocknr=8
  [  399.907471][T15342] b_state=0x00000029, b_size=512
  [  399.912506][T15342] device loop0 blocksize: 4096

This patch reduces frequency to up to once per a second, in addition to
concatenating three lines into one.

  [  399.866302][T15342] __find_get_block_slow() failed. block=1, b_blocknr=8, b_state=0x00000029, b_size=512, device loop0 blocksize: 4096
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/buffer.c

@@ -200,6 +200,7 @@ __find_get_block_slow(struct block_device *bdev, sector_t block)
 	struct buffer_head *head;
 	struct page *page;
 	int all_mapped = 1;
+	static DEFINE_RATELIMIT_STATE(last_warned, HZ, 1);
 
 	index = block >> (PAGE_SHIFT - bd_inode->i_blkbits);
 	page = find_get_page_flags(bd_mapping, index, FGP_ACCESSED);
@@ -227,14 +228,14 @@ __find_get_block_slow(struct block_device *bdev, sector_t block)
 	 * file io on the block device and getblk.  It gets dealt with
 	 * elsewhere, don't buffer_error if we had some unmapped buffers
 	 */
-	if (all_mapped) {
-		printk("__find_get_block_slow() failed. "
-			"block=%llu, b_blocknr=%llu\n",
+	ratelimit_set_flags(&last_warned, RATELIMIT_MSG_ON_RELEASE);
+	if (all_mapped && __ratelimit(&last_warned)) {
+		printk("__find_get_block_slow() failed. block=%llu, "
+		       "b_blocknr=%llu, b_state=0x%08lx, b_size=%zu, "
+		       "device %pg blocksize: %d\n",
 		       (unsigned long long)block,
-			(unsigned long long)bh->b_blocknr);
-		printk("b_state=0x%08lx, b_size=%zu\n",
-			bh->b_state, bh->b_size);
-		printk("device %pg blocksize: %d\n", bdev,
+		       (unsigned long long)bh->b_blocknr,
+		       bh->b_state, bh->b_size, bdev,
 		       1 << bd_inode->i_blkbits);
 	}
 out_unlock: