btrfs: add test case for rwf_nowait writes

btrfs: add test case for rwf_nowait writes

Test several scenarios for RWF_NOWAIT writes, to verify we don't regress
on btrfs specific behaviour (snapshots, cow files, reflinks, holes,
prealloc extent beyond eof).

We had some bugs in the past related to RWF_NOWAIT writes not failing on
btrfs when they should or failing when they shouldn't, these were fixed by
the following kernel commits:

  4b1946284dd6 ("btrfs: fix failure of RWF_NOWAIT write into prealloc extent beyond eof")
  260a63395f90 ("btrfs: fix RWF_NOWAIT write not failling when we need to cow")

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