btrfs: test fiemap on large file with extents shared through a snapshot

btrfs: test fiemap on large file with extents shared through a snapshot

Verify that fiemap correctly reports the sharedness of extents for a file
with a very large number of extents, spanning many b+tree leaves in the fs
tree, and when the file's subvolume was snapshoted.

Currently this passes on all kernel releases and its purpose is to prevent
and detect regressions in the future, as this actually happened during
recent development on the btrfs' fiemap related code. With this test we
now have better coverage for fiemap when a file is shared through a
snapshot.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>