tools lib api debugfs: Check for tracefs when reporting errors

Now that we have two mountpoints, one for debugfs and another, for
tracefs, we end up needing to check permissions for both, so, on
a system with default config we were always asking the user to
check the permission of the debugfs mountpoint, even when it was
already sufficient. Fix it.

E.g.:

  $ trace -e nanosleep usleep 1
  Error: No permissions to read /sys/kernel/debug/tracing/events/raw_syscalls/sys_(enter|exit)
  Hint:  Try 'sudo mount -o remount,mode=755 /sys/kernel/debug'

  $ sudo mount -o remount,mode=755 /sys/kernel/debug
  $ trace -e nanosleep usleep 1
  Error: No permissions to read /sys/kernel/debug/tracing/events/raw_syscalls/sys_(enter|exit)
  Hint:  Try 'sudo mount -o remount,mode=755 /sys/kernel/debug/tracing'

  $ sudo mount -o remount,mode=755 /sys/kernel/debug/tracing
  $ trace -e nanosleep usleep 1
     0.326 ( 0.061 ms): usleep/11961 nanosleep(rqtp: 0x7ffef1081c50) = 0
  $

Cc: Adrian Hunter <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Steven Rostedt <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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