NFS: Don't cleanup sysfs superblock entry if uninitialized

Its possible to end up in nfs_free_server() before the server's superblock
sysfs entry has been initialized, in which case calling kobject_put() will
emit a WARNING.  Check if the kobject has been initialized before cleaning
it up.

Fixes: 1c725118

 ("NFS: add superblock sysfs entries")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>