kernfs: remove unnecessary NULL check in __kernfs_remove()

895a068a ("kernfs: make kernfs_get_active() block if the node is
deactivated but not removed") added "struct kernfs_root *root =
kernfs_root(kn);" at the head of the function; however, the parameter
@kn is checked for later implying that the function may be called with
NULL.  This means that we may end up invoking kernfs_root() with NULL
which will oops.  None of the existing users invokes removal with NULL
@kn, so this bug doesn't actually trigger.

We can relocate kernfs_root() invocation after NULL check; however,
allowing NULL param tends to cause more confusion than actually
helping anything.  As there's no existing user, let's remove the
spurious NULL check.

This bug was detected by smatch.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>