perf/hw_breakpoint: Simplify breakpoint enable in perf_event_modify_breakpoint

perf/hw_breakpoint: Simplify breakpoint enable in perf_event_modify_breakpoint

We can safely enable the breakpoint back for both the fail and success
paths by checking only the bp->attr.disabled, which either holds the new
'requested' disabled state or the original breakpoint state.

Committer testing:

At the end of the series, the 'perf test' entry introduced as the first
patch now runs to completion without finding the fixed issues:

  # perf test "bp modify"
  62: x86 bp modify                                         : Ok
  #

In verbose mode:

  # perf test -v "bp modify"
  62: x86 bp modify                                         :
  --- start ---
  test child forked, pid 5161
  rip 5950a0, bp_1 0x5950a0
  in bp_1
  rip 5950a0, bp_1 0x5950a0
  in bp_1
  test child finished with 0
  ---- end ----
  x86 bp modify: Ok

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Milind Chabbi <chabbi.milind@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180827091228.2878-6-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>