signal/x86: Call force_sig_pkuerr from __bad_area_nosemaphore

There is only one code path that can generate a pkuerr signal.  That
code path calls __bad_area_nosemaphore and can be dectected by testing
if si_code == SEGV_PKUERR.  It can be seen from inspection that all of
the other tests in fill_sig_info_pkey are unnecessary.

Therefore call force_sig_pkuerr directly from __bad_area_semaphore and
remove fill_sig_info_pkey.

At the same time move the comment above force_sig_info_pkey into
bad_area_access_error, so that the documentation about pkey generation
races is not lost.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>