btrfs/282: skip test if /var/lib/btrfs isnt writable

btrfs/282: skip test if /var/lib/btrfs isnt writable

I run fstests in a readonly container, and accidentally uninstalled the
btrfsprogs package.  When I did, this test started faililng:

  --- btrfs/282.out
  +++ btrfs/282.out.bad
  @@ -1,3 +1,7 @@
   QA output created by 282
   wrote 2147483648/2147483648 bytes at offset 0
   XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
  +WARNING: cannot create scrub data file, mkdir /var/lib/btrfs failed: Read-only file system. Status recording disabled
  +WARNING: failed to open the progress status socket at /var/lib/btrfs/scrub.progress.3e1cf8c6-8f8f-4b51-982c-d6783b8b8825: No such file or directory. Progress cannot be queried
  +WARNING: cannot create scrub data file, mkdir /var/lib/btrfs failed: Read-only file system. Status recording disabled
  +WARNING: failed to open the progress status socket at /var/lib/btrfs/scrub.progress.3e1cf8c6-8f8f-4b51-982c-d6783b8b8825: No such file or directory. Progress cannot be queried

Skip the test if /var/lib/btrfs isn't writable, or if /var/lib isn't
writable, which means we cannot create /var/lib/btrfs.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>