blktrace: disable kill option - take 2

blktrace: disable kill option - take 2

Fixup for 513950.

Problem:
'blktrace -d <device> -k' does not kill a running
backgound trace. Executing 'blktrace -d <device> -k'
for the second time results in "BLKTRACETEARDOWN:
Invalid argument" message and then each run of
blktrace on that machine prints the following output:
BLKTRACESETUP: No such file or directory.

The bug:
The option -k results in clobbering information
about running trace by kernel (blk_trace_remove),
while resources (files open in debugfs by the running
background blktrace) are not released.

Solution:
Update documentation:
Undocument the non-working "kill" option. Advise
to send SIGINT signall via kill(1) to the running
background blktrace for its correct termination.

Signed-off-by: Edward Shishkin <edward@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>