When btrfs_get_extent() tries to get some file extent from disk, it
never populates extent_map::generation, leaving the value to be 0.

On the other hand, for extent map generated by IO, it will get its
generation properly set at finish_ordered_io()

 finish_ordered_io()
 |- unpin_extent_cache(gen = trans->transid)
    |- em->generation = gen;

[CAUSE]
Since extent_map::generation is mostly used by fsync code, and for fsync
they only care about modified extents, which all have their
em::generation > 0.

Thus it's fine to not populate em read from disk for fsync.

[CORNER CASE]
However autodefrag also relies on em::generation to determine if one
extent needs to be defragged.

This unpopulated extent_map::generation can prevent the following
autodefrag case from working:

	mkfs.btrfs -f $dev
	mount $dev $mnt -o autodefrag

	# initial write to queue the inode for autodefrag
	xfs_io -f -c "pwrite 0 4k" $mnt/file
	sync

	# Real fragmented write
	xfs_io -f -s -c "pwrite -b 4096 0 32k" $mnt/file
	sync
	echo "=== before autodefrag ==="
	xfs_io -c "fiemap -v" $mnt/file

	# Drop cache to force em to be read from disk
	echo 3 > /proc/sys/vm/drop_caches
	mount -o remount,commit=1 $mnt
	sleep 3
	sync

	echo "=== After autodefrag ==="
	xfs_io -c "fiemap -v" $mnt/file
	umount $mnt

The result looks like this:

  === before autodefrag ===
  /mnt/btrfs/file:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..15]:         26672..26687        16   0x0
     1: [16..31]:        26656..26671        16   0x0
     2: [32..47]:        26640..26655        16   0x0
     3: [48..63]:        26624..26639        16   0x1
  === After autodefrag ===
  /mnt/btrfs/file:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..15]:         26672..26687        16   0x0
     1: [16..31]:        26656..26671        16   0x0
     2: [32..47]:        26640..26655        16   0x0
     3: [48..63]:        26624..26639        16   0x1

This fragmented 32K will not be defragged by autodefrag.

[FIX]
To make things less weird, just populate extent_map::generation when
reading file extents from disk.

This would make above fragmented extents to be properly defragged:

  == before autodefrag ===
  /mnt/btrfs/file:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..15]:         26672..26687        16   0x0
     1: [16..31]:        26656..26671        16   0x0
     2: [32..47]:        26640..26655        16   0x0
     3: [48..63]:        26624..26639        16   0x1
  === After autodefrag ===
  /mnt/btrfs/file:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..63]:         26688..26751        64   0x1
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>