ppc: Clean up scanlog

Clean up the pseries scanlog driver's use of procfs:

 (1) Don't need to save the proc_dir_entry pointer as we have the filename to
     remove with.

 (2) Save the scan log buffer pointer in a static variable (there is only one
     of it) and don't save it in the PDE (which doesn't have a destructor).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
cc: Paul Mackerras <paulus@samba.org>
cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>