lib min_heap: Switch to size_t

size_t is the correct type for a count of objects that can fit in
memory: this also means heaps now have the same memory layout as darrays
(fs/bcachefs/darray.h), and darrays can be used as heaps.

Cc: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Coly Li <colyli@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

include/linux/min_heap.h

@@ -15,8 +15,8 @@
  */
 #define MIN_HEAP_PREALLOCATED(_type, _name, _nr)	\
 struct _name {	\
-	int nr;	\
-	int size;	\
+	size_t nr;	\
+	size_t size;	\
 	_type *data;	\
 	_type preallocated[_nr];	\
 }