[scudo] Return nullptr if a remap fails on linux. (#200537)

Add a check if a fixed address mmap doesn't return the expected address.

Allow a remap call to fail if the mmap fails and returns a nullptr to
the caller.

Fix a place where if remap fails in the secondary, it didn't do
anything. Now it will unmap the original entry on failure.

GitOrigin-RevId: adcd285fc4047759e4014d6568d80bb64589aded