btrfs: regression test for the clone ioctl

btrfs: regression test for the clone ioctl

This tests that we can not clone an inline extent into a non-zero file
offset. Inline extents at non-zero offsets is something btrfs is not
prepared for and results in all sorts of corruption and crashes on
future IO operations, such as the following BUG_ON() triggered by the
last write operation done by this test:

  [152154.035903] ------------[ cut here ]------------
  [152154.036424] kernel BUG at mm/page-writeback.c:2286!
  [152154.036424] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
  (...)
  [152154.036424] RIP: 0010:[<ffffffff8111a9d5>]  [<ffffffff8111a9d5>] clear_page_dirty_for_io+0x1e/0x90
  (...)
  [152154.036424] Call Trace:
  [152154.036424]  [<ffffffffa04e97c1>] lock_and_cleanup_extent_if_need+0x147/0x18d [btrfs]
  [152154.036424]  [<ffffffffa04ea82c>] __btrfs_buffered_write+0x245/0x4c8 [btrfs]
  [152154.036424]  [<ffffffffa04ed14b>] ? btrfs_file_write_iter+0x150/0x3e0 [btrfs]
  [152154.036424]  [<ffffffffa04ed15a>] ? btrfs_file_write_iter+0x15f/0x3e0 [btrfs]
  [152154.036424]  [<ffffffffa04ed2c7>] btrfs_file_write_iter+0x2cc/0x3e0 [btrfs]
  [152154.036424]  [<ffffffff81165a4a>] __vfs_write+0x7c/0xa5
  [152154.036424]  [<ffffffff81165f89>] vfs_write+0xa0/0xe4
  [152154.036424]  [<ffffffff81166855>] SyS_pwrite64+0x64/0x82
  [152154.036424]  [<ffffffff81465197>] system_call_fastpath+0x12/0x6f
  (...)
  [152154.242621] ---[ end trace e3d3376b23a57041 ]---

This issue is addressed by the following linux kernel patch for btrfs:
"Btrfs: fix file corruption after cloning inline extents".

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>