powerpc/slb: Rename all the 'slot' occurrences to 'entry'

The SLB code uses 'slot' and 'entry' interchangeably, change it to always
use 'entry'.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Rewrite change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>