Core: use ngx_palloc_small() to allocate ngx_pool_large_t.

This structure cannot be allocated as a large block anyway, otherwise that will
result in infinite recursion, since each large allocation requires to allocate
another ngx_pool_large_t.

The room for the structure is guaranteed by the NGX_MIN_POOL_SIZE constant.

src/core/ngx_palloc.c

diff --git a/src/core/ngx_palloc.c b/src/core/ngx_palloc.c
index 587a1e5..91e7cd7 100644
--- a/src/core/ngx_palloc.c
+++ b/src/core/ngx_palloc.c
@@ -231,7 +231,7 @@
         }
     }
 
-    large = ngx_palloc(pool, sizeof(ngx_pool_large_t));
+    large = ngx_palloc_small(pool, sizeof(ngx_pool_large_t), 1);
     if (large == NULL) {
         ngx_free(p);
         return NULL;
@@ -256,7 +256,7 @@
         return NULL;
     }
 
-    large = ngx_palloc(pool, sizeof(ngx_pool_large_t));
+    large = ngx_palloc_small(pool, sizeof(ngx_pool_large_t), 1);
     if (large == NULL) {
         ngx_free(p);
         return NULL;