arm64: split syscall_trace() into separate functions for enter/exit

arm64: split syscall_trace() into separate functions for enter/exit

As done in arm, this change makes it easy to confirm we invoke syscall
related hooks, including syscall tracepoint, audit and seccomp which would
be implemented later, in correct order. That is, undoing operations in the
opposite order on exit that they were done on entry.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 3157858feff89196635b01495d5ec9ebe206639e)
Signed-off-by: Mark Brown <broonie@linaro.org>
Conflicts:
	arch/arm64/kernel/ptrace.c