riscv: entry: set a0 = -ENOSYS only when syscall != -1

When we test seccomp with 6.4 kernel, we found errno has wrong value.
If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
get ENOSYS instead. We got same result with commit 9c2598d43510 ("riscv:
entry: Save a0 prior syscall_enter_from_user_mode()").

After analysing code, we think that regs->a0 = -ENOSYS should only be
executed when syscall != -1. In __seccomp_filter, when seccomp rejected
this syscall with specified errno, they will set a0 to return number as
syscall ABI, and then return -1. This return number is finally pass as
return number of syscall_enter_from_user_mode, and then is compared with
NR_syscalls after converted to ulong (so it will be ULONG_MAX). The
condition syscall < NR_syscalls will always be false, so regs->a0 = -ENOSYS
is always executed. It covered a0 set by seccomp, so we always get
ENOSYS when match seccomp RET_ERRNO rule.

Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Reported-by: Felix Yan <[email protected]>
Co-developed-by: Ruizhe Pan <[email protected]>
Signed-off-by: Ruizhe Pan <[email protected]>
Co-developed-by: Shiqi Zhang <[email protected]>
Signed-off-by: Shiqi Zhang <[email protected]>
Signed-off-by: Celeste Liu <[email protected]>
Tested-by: Felix Yan <[email protected]>
Tested-by: Emil Renner Berthing <[email protected]>
Reviewed-by: Björn Töpel <[email protected]>
Reviewed-by: Guo Ren <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

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