sparc64: Fix accidental syscall restart on child return from clone/fork/vfork.

This fixes a regression added by
238468b2

 ("[SPARC64]: Use trap type
stored in pt_regs to handle syscall restart.")

Because we now encode the "returning from syscall" status in the
pt_regs area, we have to be mindful to zap it out in the child
of a fork.

During a parallel kernel build I saw an accidental -EINTR return
from vfork() in 'make' because of this bug.

Signed-off-by: David S. Miller <davem@davemloft.net>