Btrfs: fix race between shrinking truncate and fiemap

When there is a fiemap executing in parallel with a shrinking truncate
we can end up in a situation where we have extent maps for which we no
longer have corresponding file extent items. This is generally harmless
and at the moment the only consequences are missing file extent items
representing holes after we expand the file size again after the
truncate operation removed the prealloc extent items, and stale
information for future fiemap calls (reporting extents that no longer
exist or may have been reallocated to other files for example).

Consider the following example:

1) Our inode has a size of 128KiB, one 128KiB extent at file offset 0
   and a 1MiB prealloc extent at file offset 128KiB;

2) Task A starts doing a shrinking truncate of our inode to reduce it to
   a size of 64KiB. Before it searches the subvolume tree for file
   extent items to delete, it drops all the extent maps in the range
   from 64KiB to (u64)-1 by calling btrfs_drop_extent_cache();

3) Task B starts doing a fiemap against our inode. When looking up for
   the inode's extent maps in the range from 128KiB to (u64)-1, it
   doesn't find any in the inode's extent map tree, since they were
   removed by task A.  Because it didn't find any in the extent map
   tree, it scans the inode's subvolume tree for file extent items, and
   it finds the 1MiB prealloc extent at file offset 128KiB, then it
   creates an extent map based on that file extent item and adds it to
   inode's extent map tree (this ends up being done by
   btrfs_get_extent() <- btrfs_get_extent_fiemap() <-
   get_extent_skip_holes());

4) Task A then drops the prealloc extent at file offset 128KiB and
   shrinks the 128KiB extent file offset 0 to a length of 64KiB. The
   truncation operation finishes and we end up with an extent map
   representing a 1MiB prealloc extent at file offset 128KiB, despite we
   don't have any more that extent;

After this the two types of problems we have are:

1) Future calls to fiemap always report that a 1MiB prealloc extent
   exists at file offset 128KiB. This is stale information, no longer
   correct;

2) If the size of the file is increased, by a truncate operation that
   increases the file size or by a write into a file offset > 64KiB for
   example, we end up not inserting file extent items to represent holes
   for any range between 128KiB and 128KiB + 1MiB, since the hole
   expansion function, btrfs_cont_expand() will skip hole insertion for
   any range for which an extent map exists that represents a prealloc
   extent. This causes fsck to complain about missing file extent items
   when not using the NO_HOLES feature.

The second issue could be often triggered by test case generic/561 from
fstests, which runs fsstress and duperemove in parallel, and duperemove
does frequent fiemap calls.

Essentially the problems happens because fiemap does not acquire the
inode's lock while truncate does, and fiemap locks the file range in the
inode's iotree while truncate does not. So fix the issue by making
btrfs_truncate_inode_items() lock the file range from the new file size
to (u64)-1, so that it serializes with fiemap.

CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>