arm64: Fix syscall restarting around signal suppressed by tracer

Commit 17c28958 ("arm64: Abstract syscallno manipulation") abstracts
out the pt_regs.syscallno value for a syscall cancelled by a tracer
as NO_SYSCALL, and provides helpers to set and check for this
condition.  However, the way this was implemented has the
unintended side-effect of disabling part of the syscall restart
logic.

This comes about because the second in_syscall() check in
do_signal() re-evaluates the "in a syscall" condition based on the
updated pt_regs instead of the original pt_regs.  forget_syscall()
is explicitly called prior to the second check in order to prevent
restart logic in the ret_to_user path being spuriously triggered,
which means that the second in_syscall() check always yields false.

This triggers a failure in
tools/testing/selftests/seccomp/seccomp_bpf.c, when using ptrace to
suppress a signal that interrups a nanosleep() syscall.

Misbehaviour of this type is only expected in the case where a
tracer suppresses a signal and the target process is either being
single-stepped or the interrupted syscall attempts to restart via
-ERESTARTBLOCK.

This patch restores the old behaviour by performing the
in_syscall() check only once at the start of the function.

Fixes: 17c28958 ("arm64: Abstract syscallno manipulation")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reported-by: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: <stable@vger.kernel.org> # 4.14.x-
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>