kernel/vm: Use more than one object_cache for the page mappings.

Every time a page is mapped into an area on fault, we have to
allocate a mapping object for it. While the object_cache
does have per-CPU depots, these depots only store a limited
number of items, and once they run out the object_cache's lock
must be acquired.

So, to reduce lock contention on SMP systems, create a number
of object caches corresponding to the nearest power of 2
that is equal or smaller than the count of CPUs. (We already
allocate dozens of object caches for the block allocator
no matter how many CPUs there are, so a few more depending
on CPU count shouldn't impact memory use too much. Besides,
the object_caches are wired into the low_resource system.)

This significantly reduces lock contention on SMP systems.
Same benchmark setup as yesterday (compile mime_db and relink
HaikuDepot, VMware, -j4), before:
real    0m16.981s
user    0m14.357s
sys     0m6.060s

after:
real    0m14.522s
user    0m14.194s
sys     0m4.337s

And the page_mappings object_cache locks went from having 200,000+ waits
and ~14 seconds waiting time (across all threads) down to ~900 (yes,
that's not a typo) and ~0.05s wait time (though these numbers were captured
in conjunction with the following commit.)

headers/private/kernel/vm/vm.h

@@ -17,7 +17,6 @@
 
 struct generic_io_vec;
 struct kernel_args;
-struct ObjectCache;
 struct system_memory_info;
 struct VMAddressSpace;
 struct VMArea;
@@ -48,9 +47,6 @@ struct VMPageWiringInfo;
 #define VM_MEMORY_RESERVE_SYSTEM	(VM_PAGE_RESERVE_SYSTEM * B_PAGE_SIZE)
 
 
-extern struct ObjectCache* gPageMappingsObjectCache;
-
-
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -86,6 +82,8 @@ area_id transfer_area(area_id id, void** _address, uint32 addressSpec,
 
 const char* vm_cache_type_to_string(int32 type);
 
+void vm_free_page_mapping(page_num_t page, vm_page_mapping* mapping, uint32 flags);
+
 status_t vm_prepare_kernel_area_debug_protection(area_id id, void** cookie);
 status_t vm_set_kernel_area_debug_protection(void* cookie, void* _address,
 			size_t size, uint32 protection);