rust_task::block_locked and rust_task::must_fail_unlocked naming inconsistency

[bblum]

Both of them are "inner" functions that are usually called by wrappers which take a lock around the call, but sometimes can be called directly if the lock is already held.

"block_locked" means "call this when already locked";
"must_fail_unlocked" means "this function doesn't do any locking"

But they mean the same thing. Clean up their names.