kernfs: kvmalloc xattr value instead of kmalloc

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 <[email protected]>
Acked-by: Chris Down <[email protected]>
Reviewed-by: Andreas Dilger <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>