performance/io-cache: fix double free.
performance/io-cache: fix double free.

- the destroyer function passed to rbthash_table_init is used to free the data.
    The data being inserted is page and it is destroyed in ioc_page_destroy. Hence
    no destroyer function needs to be passed to rbthash_table_init.

Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 335 (Io-cache optimization)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=335

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster