ppc: Export tlb_44x_hwater for KVM

PowerPC 440 KVM needs to know how many TLB entries are used for the host kernel
linear mapping (it does not modify these mappings when switching between guest
and host execution).

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Avi Kivity <avi@qumranet.com>