btrfs: fix race when detecting delalloc ranges during fiemap

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix race when detecting delalloc ranges during fiemap

For fiemap we recently stopped locking the target extent range for the
whole duration of the fiemap call, in order to avoid a deadlock in a
scenario where the fiemap buffer happens to be a memory mapped range of
the same file. This use case is very unlikely to be useful in practice but
it may be triggered by fuzz testing (syzbot, etc).

This however introduced a race that makes us miss delalloc ranges for
file regions that are currently holes, so the caller of fiemap will not
be aware that there's data for some file regions. This can be quite
serious for some use cases - for example in coreutils versions before 9.0,
the cp program used fiemap to detect holes and data in the source file,
copying only regions with data (extents or delalloc) from the source file
to the destination file in order to preserve holes (see the documentation
for its --sparse command line option). This means that if cp was used
with a source file that had delalloc in a hole, the destination file could
end up without that data, which is effectively a data loss issue, if it
happened to hit the race described below.

The race happens like this:

1. Fiemap is called, without the FIEMAP_FLAG_SYNC flag, for a file that
   has delalloc in the file range [64M, 65M[, which is currently a hole;

2. Fiemap locks the inode in shared mode, then starts iterating the
   inode's subvolume t…