vm: add alignment parameter to cache allocator

caches with object size N, where N is a power of two, will naturally
align their objects on N-byte boundaries.

all other caches will align to an 8-byte boundary by default.

include/socks/vm.h

@@ -137,9 +137,11 @@ typedef struct vm_cache {
 	unsigned int c_obj_size;
 	/* combined size of vm_slab_t and the freelist */
 	unsigned int c_hdr_size;
+	/* power of 2 alignment for objects returned from the cache */
+	unsigned int c_align;
 	/* offset from one object to the next in a slab.
-	   this may be different from c_obj_size as
-	   we enforce a 16-byte alignment on allocated objects */
+	   this may be different from c_obj_size depending
+	   on the alignment settings for this cache. */
 	unsigned int c_stride;
 	/* size of page used for slabs */
 	unsigned int c_page_order;