net/sunrpc: fix potential memory leaks in rpc_sysfs_xprt_state_change()

In the Linux kernel, the following vulnerability has been resolved: net/sunrpc: fix potential memory leaks in rpc_sysfs_xprt_state_change() The issue happens on some error handling paths. When the function fails to grab the object `xprt`, it simply returns 0, forgetting to decrease the reference count of another object `xps`, which is increased by rpc_sysfs_xprt_kobj_get_xprt_switch(), causing refcount leaks. Also, the function forgets to check whether `xps` is valid before using it, which may result in NULL-dereferencing issues. Fix it by adding proper error handling code when either `xprt` or `xps` is NULL.