cifs: Change from mempool_destroy to mempool_exit for request pools

insmod followed by rmmod was oopsing with the new mempools cifs request patch

Fixes: edea94a6

 ("cifs: Add mempools for cifs_io_request and cifs_io_subrequest structs")
Suggested-by: David Howells <dhowells@redhat.com>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>