Expand INIT_TASK() in init/init_task.c and remove

It's no longer necessary to have an INIT_TASK() macro, and this can be
expanded into the one place it is now used and removed.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Tony Luck <tony.luck@intel.com>
Tested-by: Will Deacon <will.deacon@arm.com> (arm64)
Tested-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>