futex: fix dead code in attach_to_pi_owner()

This patch comes directly from an origin patch (commit
91509e84) in v4.9.
And it is part of a full patch which was originally back-ported
to v4.14 as commit e6e00df1

The handle_exit_race() function is defined in commit 9c3f3986
 ("futex: Cure exit race"), which never returns -EBUSY. This results
in a small piece of dead code in the attach_to_pi_owner() function:

	int ret = handle_exit_race(uaddr, uval, p); /* Never return -EBUSY */
	...
	if (ret == -EBUSY)
		*exiting = p; /* dead code */

The return value -EBUSY is added to handle_exit_race() in upsteam
commit ac31c7ff ("futex: Provide distinct return value when
owner is exiting"). This commit was incorporated into v4.9.255, before
the function handle_exit_race() was introduced, whitout Modify
handle_exit_race().

To fix dead code, extract the change of handle_exit_race() from
commit ac31c7ff ("futex: Provide distinct return value when owner
 is exiting"), re-incorporated.

Lee writes:

This commit takes the remaining functional snippet of:

 ac31c7ff ("futex: Provide distinct return value when owner is exiting")

... and is the correct fix for this issue.

Fixes: 9c3f3986

 ("futex: Cure exit race")
Cc: stable@vger.kernel.org # v4.9.258
Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Reviewed-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>