kernfs: checking for IS_ERR() instead of NULL

The kernfs_get_inode() returns NULL on error, it never returns error
pointers.

Fixes: aa818825 ("kernfs: add exportfs operations")
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>