ptrace: Remove unmatched preempt_count() issue in ptrace_stop

Upstream commit 53da1d94 ("fix ptrace slowness") introduced
preempt_disable() and preempt_enable_no_resched() in function
ptrace_stop(), and they should be used together.

In RT kernel, commit 500d5c546e46 ("signal: Revert ptrace preempt
magic") reverted commit 53da1d94. To make Yocto RT kernel the
same as upstream RT kernel, remove preempt_enable_no_resched(), in
stread of adding preempt_disable().

Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>