Fixed unix ngx_write_chain_to_file() to return total bytes written.

Previously result of last iteration's writev() was returned.  This was
unnoticed as return value was only used if chain contained only one or
two buffers.

src/os/unix/ngx_files.c

diff --git a/src/os/unix/ngx_files.c b/src/os/unix/ngx_files.c
index 89ab8d6..4bfde44 100644
--- a/src/os/unix/ngx_files.c
+++ b/src/os/unix/ngx_files.c
@@ -153,7 +153,7 @@
 {
     u_char        *prev;
     size_t         size;
-    ssize_t        n;
+    ssize_t        total, n;
     ngx_array_t    vec;
     struct iovec  *iov, iovs[NGX_IOVS];
 
@@ -165,6 +165,8 @@
                               offset);
     }
 
+    total = 0;
+
     vec.elts = iovs;
     vec.size = sizeof(struct iovec);
     vec.nalloc = NGX_IOVS;
@@ -233,10 +235,11 @@
 
         file->sys_offset += n;
         file->offset += n;
+        total += n;
 
     } while (cl);
 
-    return n;
+    return total;
 }