powerpc: Fix various syscall/signal/swapcontext bugs
A careful reading of the recent changes to the system call entry/exit
paths revealed several problems, plus some things that could be
simplified and improved:
* 32-bit wasn't testing the _TIF_NOERROR bit in the syscall fast exit
path, so it was only doing anything with it once it saw some other
bit being set. In other words, the noerror behaviour would apply to
the next system call where we had to reschedule or deliver a signal,
which is not necessarily the current system call.
* 32-bit wasn't doing the call to ptrace_notify in the syscall exit
path when the _TIF_SINGLESTEP bit was set.
* _TIF_RESTOREALL was in both _TIF_USER_WORK_MASK and
_TIF_PERSYSCALL_MASK, which is odd since _TIF_RESTOREALL is only set
by system calls. I took it out of _TIF_USER_WORK_MASK.
* On 64-bit, _TIF_RESTOREALL wasn't causing the non-volatile registers
to be restored (unless perhaps a signal was delivered or the syscall
was traced or single-stepped). Thus the non-volatile registers
weren't restored on exit from a signal handler. We probably got
away with it mostly because signal handlers written in C wouldn't
alter the non-volatile registers.
* On 32-bit I simplified the code and made it more like 64-bit by
making the syscall exit path jump to ret_from_except to handle
preemption and signal delivery.
* 32-bit was calling do_signal unnecessarily when _TIF_RESTOREALL was
set - but I think because of that 32-bit was actually restoring the
non-volatile registers on exit from a signal handler.
* I changed the order of enabling interrupts and saving the
non-volatile registers before calling do_syscall_trace_leave; now we
enable interrupts first.
Signed-off-by: 
Paul Mackerras <paulus@samba.org>
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