Fix the hang issue when unload the fastsocket with selinux enabled

Root reason:
There will be duplicated security inode free when unload the fastsocket.
When load the fastsocket, kernel will allocate one non-socket inode.
When unload the fastsocket, kernel invokes destroy_inode to destory
the inode.
As a result, there are duplicated security free.
1. destroy_inode->__destroy_inode->security_inode_free;
2. destroy_inode->fastsock_destroy_inode->security_inode_free;

Fix:
Add S_ISSOCK check before security_inode_free in fastsock_destroy_inode.
1. For the non-socket inode, it could avoid duplicated security_inode_free call;
2. For fastsocket inode, it still invokes security_inode_free;