kern___realpathat(): fix uninitialized memory read

[kib]

Summary

kern___realpathat(): style

kern___realpathat(): do not copyout past end of string    
Reported and tested by:    pho

kern___realpathat(): honor uio_seg argument

[kib]

Just realized that this is a kernel memory exposure.

[markj]

In D47739#1088673, @kib wrote:

Just realized that this is a kernel memory exposure.

Yes :(

It looks like the problem can happen "only" when a nullfs file mount is present?

[kib]

In D47739#1088711, @markj wrote:

In D47739#1088673, @kib wrote:

Just realized that this is a kernel memory exposure.

Yes :(

It looks like the problem can happen "only" when a nullfs file mount is present?

Yes, the mount over a file.