Clean up slb_lookup() function

Clean up slb_lookup() function

The slb_lookup() function, used in the ppc translation path returns a
number of slb entry fields in reference parameters.  However, only one
of the two callers of slb_lookup() actually wants this information.

This patch, therefore, makes slb_lookup() return a simple pointer to the
located SLB entry (or NULL), and the caller which needs the fields can
extract them itself.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>