Make n_slbs public in a more straightforward way. Some platforms (like

PowerNV) use firmware-assisted mechanisms to discover it and need access
to the variable.

MFC after:	3 weeks

sys/powerpc/aim/aim_machdep.c

@@ -130,7 +130,7 @@ __FBSDID("$FreeBSD$");
 #include <dev/ofw/openfirm.h>
 
 #ifdef __powerpc64__
-extern int n_slbs;
+#include "mmu_oea64.h"
 #endif
 
 #ifndef __powerpc64__

sys/powerpc/aim/mmu_oea64.h

@@ -81,6 +81,7 @@ extern int		moea64_large_page_shift;
 extern uint64_t		moea64_large_page_size;
 extern u_int		moea64_pteg_count;
 extern u_int		moea64_pteg_mask;
+extern int		n_slbs;
 
 #endif /* _POWERPC_AIM_MMU_OEA64_H */