btrfs: fix too long loop when defragging a 1 byte file

When attempting to defrag a file with a single byte, we can end up in a
too long loop, which is nearly infinite because at btrfs_defrag_file()
we end up with the variable last_byte assigned with a value of
18446744073709551615 (which is (u64)-1). The problem comes from the fact
we end up doing:

    last_byte = round_up(last_byte, fs_info->sectorsize) - 1;

So if last_byte was assigned 0, which is i_size - 1, we underflow and
end up with the value 18446744073709551615.

This is trivial to reproduce and the following script triggers it:

  $ cat test.sh
  #!/bin/bash

  DEV=/dev/sdj
  MNT=/mnt/sdj

  mkfs.btrfs -f $DEV
  mount $DEV $MNT

  echo -n "X" > $MNT/foobar

  btrfs filesystem defragment $MNT/foobar

  umount $MNT

So fix this by not decrementing last_byte by 1 before doing the sector
size round up. Also, to make it easier to follow, make the round up right
after computing last_byte.

Reported-by: Anthony Ruhier <aruhier@mailbox.org>
Fixes: 7b50803 ("btrfs: defrag: use defrag_one_cluster() to implement btrfs_defrag_file()")
Link: https://lore.kernel.org/linux-btrfs/0a269612-e43f-da22-c5bc-b34b1b56ebe8@mailbox.org/
CC: stable@vger.kernel.org # 5.16
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/ioctl.c

@@ -1518,12 +1518,16 @@ int btrfs_defrag_file(struct inode *inode, struct file_ra_state *ra,
 
 	if (range->start + range->len > range->start) {
 		/* Got a specific range */
-		last_byte = min(isize, range->start + range->len) - 1;
+		last_byte = min(isize, range->start + range->len);
 	} else {
 		/* Defrag until file end */
-		last_byte = isize - 1;
+		last_byte = isize;
 	}
 
+	/* Align the range */
+	cur = round_down(range->start, fs_info->sectorsize);
+	last_byte = round_up(last_byte, fs_info->sectorsize) - 1;
+
 	/*
 	 * If we were not given a ra, allocate a readahead context. As
 	 * readahead is just an optimization, defrag will work without it so
@@ -1536,10 +1540,6 @@ int btrfs_defrag_file(struct inode *inode, struct file_ra_state *ra,
 			file_ra_state_init(ra, inode->i_mapping);
 	}
 
-	/* Align the range */
-	cur = round_down(range->start, fs_info->sectorsize);
-	last_byte = round_up(last_byte, fs_info->sectorsize) - 1;
-
 	while (cur < last_byte) {
 		u64 cluster_end;