syz-manager: handle extra coverage in /syscalls page
syz-manager: handle extra coverage in /syscalls page

Extra coverage is attributed to a fake ".extra" syscall, which is not present
in SyscallMap. As syz-manager is not aware of this, it fails to render the
/syscalls page, when remote coverage is present.

Handle fake syscalls gracefully during /syscalls page rendering.

Closes #2725.

google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller