Kernel: Make Process::current() return a Process& instead of Process*

This has several benefits:
1) We no longer just blindly derefence a null pointer in various places
2) We will get nicer runtime error messages if the current process does
turn out to be null in the call location
3) GCC no longer complains about possible nullptr dereferences when
compiling without KUBSAN

Kernel/Syscalls/fcntl.cpp

@@ -47,7 +47,7 @@ KResultOr<FlatPtr> Process::sys$fcntl(int fd, int cmd, u32 arg)
     case F_GETLK:
         return description->get_flock(Userspace<flock*>(arg));
     case F_SETLK:
-        return description->apply_flock(*Process::current(), Userspace<const flock*>(arg));
+        return description->apply_flock(Process::current(), Userspace<const flock*>(arg));
     default:
         return EINVAL;
     }