fellow_cache: add fellow_cache_seglist_capaity utility

src/fellow_cache.c

@@ -2480,6 +2480,15 @@ fellow_busy_fill_segmem(struct buddy_reqs *reqs, const void *priv)
 		AN(buddy_req_page(reqs, bits, 0));
 }
 
+static size_t
+fellow_cache_seglist_capacity(unsigned bits)
+{
+	size_t segs = fellow_cache_seglist_fit((size_t)1 << bits);
+	if (segs > FELLOW_DISK_SEGLIST_MAX_SEGS)
+		segs = FELLOW_DISK_SEGLIST_MAX_SEGS;
+	return (segs);
+}
+
 struct fellow_cache_res
 fellow_busy_obj_alloc(struct fellow_cache *fc,
     struct fellow_cache_obj **fcop, uintptr_t *priv2,
@@ -2774,9 +2783,7 @@ fellow_busy_body_seglist_size_strategy(size_t segs, unsigned chunk_exponent)
 	else if (bits > chunk_exponent)
 		bits = chunk_exponent;
 
-	segs = fellow_cache_seglist_fit((size_t)1 << bits);
-	if (segs > FELLOW_DISK_SEGLIST_MAX_SEGS)
-		segs = FELLOW_DISK_SEGLIST_MAX_SEGS;
+	segs = fellow_cache_seglist_capacity(bits);
 	assert(segs <= UINT16_MAX);
 	return ((uint16_t)segs);
 }