Update flashcache_main.c

I think the pair of brackets are necessary.
In the case of writecache, when a error occurs, 
if the cache_mode is FLASHCACHE_WRITE_THROUGH,we should add 1 to both of ssd_write_errors and disk_write_errors(in the current version of flashcache).
If the cache_mode is FLASHCACHE_WRITE_BACK,we should only add 1 to ssd_write_errors.
Without the pair of brackets,no matter which cache_mode it is,both of ssd_write_errors and disk_write_errors will be added by 1,then the conditional judgement " if (dmc->cache_mode == FLASHCACHE_WRITE_THROUGH) " makes no sense.