If TIF_SYSCALL_TRACE or TIF_SINGLESTEP is set while inside a syscall,
the path back to user mode should get to syscall_trace_leave.

This does happen in most circumstances.  The exception to this is on
the 64-bit syscall fastpath, when no such flag was set on syscall
entry and nothing else has punted it off the fastpath for exit.  That
one exit fastpath fails to check for _TIF_WORK_SYSCALL_EXIT flags.
This makes the behavior inconsistent with what 32-bit tasks see and
what the native 32-bit kernel always does, and what 64-bit tasks see
in all cases where the iret path is taken anyhow.

Perhaps the only example that is affected is a ptrace stop inside
do_fork (for PTRACE_O_TRACE{CLONE,FORK,VFORK,VFORKDONE}).  Other
syscalls with internal ptrace stop points (execve) already take the
iret exit path for unrelated reasons.

Test cases for both PTRACE_SYSCALL and PTRACE_SINGLESTEP variants are at:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/~checkout~/tests/ptrace-tests/tests/syscall-from-clone.c?cvsroot=systemtap
http://sources.redhat.com/cgi-bin/cvsweb.cgi/~checkout~/tests/ptrace-tests/tests/step-from-clone.c?cvsroot=systemtap

There was no special benefit to the sysret path's special path to call
do_notify_resume, because it always takes the iret exit path at the end.
So this change just makes the sysret exit path join the iret exit path
for all the signals and ptrace cases.  The fastpath still applies to
the plain syscall-audit and resched cases.
Signed-off-by: Roland McGrath <roland@redhat.com>
CC: Oleg Nesterov <oleg@redhat.com>