btrfs: skip backref walking during fiemap if we know the leaf is shared

During fiemap, when checking if a data extent is shared we are doing the
backref walking even if we already know the leaf is shared, which is a
waste of time since if the leaf shared then the data extent is also
shared. So skip the backref walking when we know we are in a shared leaf.

The following test was measures the gains for a case where all leaves
are shared due to a snapshot:

   $ cat test.sh
   #!/bin/bash

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

   umount $DEV &> /dev/null
   mkfs.btrfs -f $DEV
   # Use compression to quickly create files with a lot of extents
   # (each with a size of 128K).
   mount -o compress=lzo $DEV $MNT

   # 40G gives 327680 extents, each with a size of 128K.
   xfs_io -f -c "pwrite -S 0xab -b 1M 0 40G" $MNT/foobar

   # Add some more files to increase the size of the fs and extent
   # trees (in the real world there's a lot of files and extents
   # from other files).
   xfs_io -f -c "pwrite -S 0xcd -b 1M 0 20G" $MNT/file1
   xfs_io -f -c "pwrite -S 0xef -b 1M 0 20G" $MNT/file2
   xfs_io -f -c "pwrite -S 0x73 -b 1M 0 20G" $MNT/file3

   # Create a snapshot so all the extents become indirectly shared
   # through subtrees, with a generation less than or equals to the
   # generation used to create the snapshot.
   btrfs subvolume snapshot -r $MNT $MNT/snap1

   # Unmount and mount again to clear cached metadata.
   umount $MNT
   mount -o compress=lzo $DEV $MNT

   start=$(date +%s%N)
   # The filefrag tool  uses the fiemap ioctl.
   filefrag $MNT/foobar
   end=$(date +%s%N)
   dur=$(( (end - start) / 1000000 ))
   echo "fiemap took $dur milliseconds (metadata not cached)"
   echo

   start=$(date +%s%N)
   filefrag $MNT/foobar
   end=$(date +%s%N)
   dur=$(( (end - start) / 1000000 ))
   echo "fiemap took $dur milliseconds (metadata cached)"

   umount $MNT

The results were the following on a non-debug kernel (Debian's default
kernel config).

Before this patch:

   (...)
   /mnt/sdi/foobar: 327680 extents found
   fiemap took 1821 milliseconds (metadata not cached)

   /mnt/sdi/foobar: 327680 extents found
   fiemap took 399 milliseconds (metadata cached)

After this patch:

   (...)
   /mnt/sdi/foobar: 327680 extents found
   fiemap took 591 milliseconds (metadata not cached)

   /mnt/sdi/foobar: 327680 extents found
   fiemap took 123 milliseconds (metadata cached)

That's a speedup of 3.1x and 3.2x.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>