btrfs: add test for btrfs send with long paths

btrfs: add test for btrfs send with long paths

Regression test for btrfs send where long paths (exceeding 230 characters)
made send produce paths with random characters from a memory buffer returned
by kmalloc, as send forgot to populate the new buffer with the path string.

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

   Btrfs: send, fix corrupted path strings for long paths

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>