btrfs: test send with prealloc extent beyond EOF and hole punching

btrfs: test send with prealloc extent beyond EOF and hole punching

Test that an incremental send operation produces correct results if
a file that has a prealloc (unwritten) extent beyond its EOF gets a
hole punched in a section of that prealloc extent.

This test is motivated by a bug found in btrfs which is fixed by a
patch for the linux kernel titled:

 "Btrfs: send, fix incorrect file layout after hole punching beyond eof"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>