xattr: fix uninitialized out-param

`removed_sized` isn't correctly initialized (as the doc comment
suggests) on memory allocation failures. Fix by moving initialization up
a bit.

Fixes: 0c47383b ("kernfs: Add option to enable user xattrs")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Tejun Heo <tj@kernel.org>