btrfs: add regression test for send with extrefs

btrfs: add regression test for send with extrefs

Regression for btrfs send when an inode only has extended references
associated to it (no regular references present). This used to cause
incorrect access to a b+tree leaf, where an extended reference item
was accessed as if it were a regular reference item, causing unexpected
and unpredictable behaviour such as producing a random/weird path string
or a crash.

This issue is fixed by the following linux kernel btrfs patch:

   Btrfs: send, fix incorrect ref access when using extrefs

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>