[POWERPC] scanlog_init cleanup and minor fixes

scanlog_init() could use some love.

* properly return -ENODEV if this system doesn't support scan-log-dump
* don't printk if scan-log-dump not present; only older systems have it
* convert from create_proc_entry() to preferred proc_create()
* allocate zeroed data buffer
* fix potential memory leak of ent->data on failed create_proc_entry()
* simplify control flow

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>