SUNRPC: Check if the xprt is connected before handling sysfs reads

xprts don't immediately reconnect when changing the "dstaddr" property,
instead this gets handled the next time an operation uses the transport.
This could lead to NULL pointer dereferences when trying to read sysfs
files between the disconnect and reconnect operations. Fix this by
returning an error if the xprt is not connected.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>