kernfs: kvmalloc xattr value instead of kmalloc

xattr values have a 64k maximum size. This can result in an order 4
kmalloc request which can be difficult to fulfill. Since xattrs do not
need physically contiguous memory, we can switch to kvmalloc and not
have to worry about higher order allocations failing.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Acked-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>