PM: wakeup: simplify the output logic of pm_show_wakelocks()

The buffer handling in pm_show_wakelocks() is tricky, and hopefully
correct.  Ensure it really is correct by using sysfs_emit_at() which
handles all of the tricky string handling logic in a PAGE_SIZE buffer
for us automatically as this is a sysfs file being read from.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>