sunrpc: Fix error handling in rpc_sysfs_xprt_switch_add_xprt_store()

sunrpc: Fix error handling in rpc_sysfs_xprt_switch_add_xprt_store()

xprt_create_transport() never returns NULL, only valid pointers or
error pointers. Using IS_ERR_OR_NULL() is incorrect, and PTR_ERR(NULL)
would return 0, which indicates EOF in a sysfs store function.

Fix this by using IS_ERR() instead of IS_ERR_OR_NULL().

Fixes: df210d9b0951 ("sunrpc: Add a sysfs file for adding a new xprt")
Signed-off-by: Hongling Zeng <zenghongling@kylinos.cn>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>