Revert "ANDROID: Re-enable fast mremap and fix UAF with SPF"

	read_lock(&mm->mm_rb_lock);

	vma = __find_vma(mm, addr);

	/*

	 * If there is a concurrent fast mremap, bail out since the entire

	 * PMD/PUD subtree may have been remapped.

	 *

	 * This is usually safe for conventional mremap since it takes the

	 * PTE locks as does SPF. However fast mremap only takes the lock

	 * at the PMD/PUD level which is ok as it is done with the mmap

	 * write lock held. But since SPF, as the term implies forgoes,

	 * taking the mmap read lock and also cannot take PTL lock at the

	 * larger PMD/PUD granualrity, since it would introduce huge

	 * contention in the page fault path; fall back to regular fault

	 * handling.

	 */

	if (vma && !atomic_inc_unless_negative(&vma->vm_ref_count))

		vma = NULL;

	if (vma)

		atomic_inc(&vma->vm_ref_count);

	read_unlock(&mm->mm_rb_lock);

	return vma;