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.