Fix on-disk cache size calculation.

Since the initial release, recorded on-disk cache size was
decreased twice for purged content (once during cache purge
and once during subsequent cache update).
This resulted in recorded on-disk cache size being much
smaller than in reality, which could lead to on-disk cache
outgrowing defined "max_size" parameter.

Based on patch from Pyry Hakulinen.

Change-Id: I13d99311d01c63f14362d995ff8dc7654ea78e0d
Signed-off-by: Piotr Sikora <piotr.sikora@frickle.com>

Author: PiotrSikora

ngx_cache_purge_module.c

@@ -757,8 +757,10 @@ ngx_http_file_cache_purge(ngx_http_request_t *r)
 
 #  if defined(nginx_version) && (nginx_version >= 1000001)
     cache->sh->size -= c->node->fs_size;
+    c->node->fs_size = 0;
 #  else
     cache->sh->size -= (c->node->length + cache->bsize - 1) / cache->bsize;
+    c->node->length = 0;
 #  endif
 
     c->node->exists = 0;