MFC:	sys/kern/kern_fork.c	1.259
MFC:	sys/kern/kern_fork.c	1.259

- Use suser_cred(9) instead of checking cr_ruid directly.
- For privileged processes safe two mutex operations.

We may want to consider if this is good idea to use SUSER_ALLOWJAIL here,
but for now I didn't wanted to change the original behaviour.

Reviewed by:	rwatson

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