generic/471: adapt test when running on btrfs to avoid failure on RWF_NOWAIT write

generic/471: adapt test when running on btrfs to avoid failure on RWF_NOWAIT write

This test currently always fails on btrfs:

generic/471 2s ... - output mismatch (see ...results//generic/471.out.bad)
    --- tests/generic/471.out   2020-06-10 19:29:03.850519863 +0100
    +++ /home/fdmanana/git/hub/xfstests/results//generic/471.out.bad   ...
    @@ -2,12 +2,10 @@
     pwrite: Resource temporarily unavailable
     wrote 8388608/8388608 bytes at offset 0
     XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
    -RWF_NOWAIT time is within limits.
    +pwrite: Resource temporarily unavailable
    +(standard_in) 1: syntax error
    +RWF_NOWAIT took  seconds

This is because btrfs is a COW filesystem and an attempt to write into a
previously written file range allocating a new extent (or multiple).
The only exceptions are when attempting to write to a file range with a
preallocated/unwritten extent or when writing to a NOCOW file that has
extents allocated in the target range already.

The test currently expects that writing into a previously written file
range succeeds, but that is not true on btrfs since we are not dealing
with a NOCOW file. So to make the test pass on btrfs, set the NOCOW bit
on the file when the filesystem is btrfs.

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