Fix on-disk cache size calculation.
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 <321b1a11e6b8c7ca12f1b0b438664e52a48b555e@frickle.com>

ainoya/ngx_cache_purge,FRiCKLE/ngx_cache_purge,ainoya/ngx_cache_purge,lilien1010/ngx_cache_purge,FRiCKLE/ngx_cache_purge,lilien1010/ngx_cache_purge