btrfs-progs: build: detect fiemap shared flag but don't fail build

btrfs-progs: build: detect fiemap shared flag but don't fail build

The FIEMAP_EXTENT_SHARED fiemap flag was introduced in 2.6.33. If the
headers do not provide the definition, the build will fail. The support
of the fiemap sharing depends on the running kernel. There are still
systems with 2.6.32 kernel headers but running newer versions.

To support such environment, don't fail build, provide own defintion of
the structure and detect if there's an old kernel in use in the relevant
command (btrfs fi du).

Reported-by: Abhay Sachan <lkp.abhay@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>