btrfs/081: fix killing of reader loop subshell

btrfs/081: fix killing of reader loop subshell

The test creates a subshell that keeps running the 'cat' command
against a test file in an infinite loop, and after it kills the
subshell it unmounts the filesystem, after which point any 'cat'
subcommand that runs after or at that time will fail resulting in an
unexpected golden output:

  $ ./check btrfs/081
  btrfs/081 3s ... - output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/081.out.bad)
      --- tests/btrfs/081.out	2018-09-16 21:30:48.501104179 +0100
      +++ /home/fdmanana/git/hub/xfstests/results//btrfs/081.out.bad	2019-01-24 20:36:18.989746185 +0000
      @@ -206,5 +206,6 @@
       Verifying file digests after cloning
       14968c092c68e32fa35e776392d14523  SCRATCH_MNT/foo
       14968c092c68e32fa35e776392d14523  SCRATCH_MNT/bar
      +cat: /mnt/scratch/bar: No such file or directory
       Verifying target file digest after umount + mount
       14968c092c68e32fa35e776392d14523  SCRATCH_MNT/bar
      ...
      (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/081.out /home/fdmanana/git/hub/xfstests/results//btrfs/081.out.bad'  to see the entire diff)
  Ran: btrfs/081
  Failures: btrfs/081
  Failed 1 of 1 tests

Fix that by adding a proper trap to the reader loop function so that
the subshell waits for executed 'cat' commands when it receives
SIGTERM.

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