Btrfs-progs: fix restore command leaving corrupted files

Btrfs-progs: fix restore command leaving corrupted files

When there are files that have parts shared with snapshots, the
restore command was incorrectly restoring them, as it was not
taking into account the offset and number of bytes fields from
the file extent item. Besides leaving the recovered file corrupt,
it was also inneficient as it read and wrote more data than needed
(with each extent copy overwriting portions of the one previously
written).

The following steps show how to reproduce this corruption issue:

$ mkfs.btrfs -f  /dev/sdb3
$ mount /dev/sdb3 /mnt/btrfs
$ perl -e '$d = "\x41" . ("\x00" x (1024*1024+349)); open($f,">","/mnt/btrfs/foobar"); print $f $d; close($f);'
$ du -b /mnt/btrfs/foobar
1048926 /mnt/btrfs/foobar
$ md5sum /mnt/btrfs/foobar
f9f778f3a7410c40e4ed104a3a63c3c4  /mnt/btrfs/foobar

$ btrfs subvolume snapshot /mnt/btrfs /mnt/btrfs/my_snap
$ perl -e 'open($f, "+<", "/mnt/btrfs/foobar"); seek($f, 4096, 0); print $f "\xff"; close($f);'
$ md5sum /mnt/btrfs/foobar
b983fcefd4622a03a78936484c40272b  /mnt/btrfs/foobar
$ umount /mnt/btrfs

$ btrfs restore /dev/sdb3 /tmp/copy
$ du -b /tmp/copy/foobar
1048926 /tmp/copy/foobar
$ md5sum /tmp/copy/foobar
88db338cbc1c44dfabae083f1ce642d5  /tmp/copy/foobar
$ od -t x1 -j 8192 -N 4 /tmp/copy/foobar
0020000 41 00 00 00
0020004
$ mount /dev/sdb3 /mnt/btrfs
$ od -t x1 -j 8192 -N 4 /mnt/btrfs/foobar
0020000 00 00 00 00
0020004
$ md5sum /mnt/btrfs/foobar
b983fcefd4622a03a78936484c40272b  /mnt/btrfs/foobar

Tested this change with zlib, lzo compression and file sizes larger
than 1GiB, and found no regression or other corruption issues (so far
at least).

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>