SUNRPC: Fix null-ptr-deref when xps sysfs alloc failed

There is a null-ptr-deref when xps sysfs alloc failed:
  BUG: KASAN: null-ptr-deref in sysfs_do_create_link_sd+0x40/0xd0
  Read of size 8 at addr 0000000000000030 by task gssproxy/457

  CPU: 5 PID: 457 Comm: gssproxy Not tainted 6.0.0-09040-g02357b27ee03 #9
  Call Trace:
   <TASK>
   dump_stack_lvl+0x34/0x44
   kasan_report+0xa3/0x120
   sysfs_do_create_link_sd+0x40/0xd0
   rpc_sysfs_client_setup+0x161/0x1b0
   rpc_new_client+0x3fc/0x6e0
   rpc_create_xprt+0x71/0x220
   rpc_create+0x1d4/0x350
   gssp_rpc_create+0xc3/0x160
   set_gssp_clnt+0xbc/0x140
   write_gssp+0x116/0x1a0
   proc_reg_write+0xd6/0x130
   vfs_write+0x177/0x690
   ksys_write+0xb9/0x150
   do_syscall_64+0x35/0x80
   entry_SYSCALL_64_after_hwframe+0x46/0xb0

When the xprt_switch sysfs alloc failed, should not add xprt and
switch sysfs to it, otherwise, maybe null-ptr-deref; also initialize
the 'xps_sysfs' to NULL to avoid oops when destroy it.

Fixes: 2a338a54 ("sunrpc: add a symlink from rpc-client directory to the xprt_switch")
Fixes: d408ebe0 ("sunrpc: add add sysfs directory per xprt under each xprt_switch")
Fixes: baea9944 ("sunrpc: add xprt_switch direcotry to sunrpc's sysfs")
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>