nommu: remove a superfluous check of vm_region::vm_usage

In split_vma(), there's no need to check if the VMA being split has a
region that's in use by more than one VMA because:

(1) The preceding test prohibits splitting of non-anonymous VMAs and regions
(eg: file or chardev backed VMAs).

(2) Anonymous regions can't be mapped multiple times because there's no handle
by which to refer to the already existing region.

(3) If a VMA has previously been split, then the region backing it has also
been split into two regions, each of usage 1.

Signed-off-by: David Howells <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Cc: Greg Ungerer <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/nommu.c

···
 
 	kenter("");
 
-	/* we're only permitted to split anonymous regions that have a single
-	 * owner */
-	if (vma->vm_file ||
-	    vma->vm_region->vm_usage != 1)
+	/* we're only permitted to split anonymous regions (these should have
+	 * only a single usage on the region) */
+	if (vma->vm_file)
 		return -ENOMEM;
 
 	if (mm->map_count >= sysctl_max_map_count)