Slab: fixed the number of pages calculation.

When estimating the number of pages, do not count memory for slots.
In some cases this gives one extra usable memory page.

src/core/ngx_slab.c

diff --git a/src/core/ngx_slab.c b/src/core/ngx_slab.c
index 4357e8d..b33f43f 100644
--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -120,6 +120,7 @@
     }
 
     p += n * sizeof(ngx_slab_page_t);
+    size -= n * sizeof(ngx_slab_page_t);
 
     pages = (ngx_uint_t) (size / (ngx_pagesize + sizeof(ngx_slab_page_t)));