md: Relax minimum size restrictions on chunk_size.

Currently, the 'chunk_size' of an array must be at-least PAGE_SIZE.

This makes moving an array to a machine with a larger PAGE_SIZE, or
changing the kernel to use a larger PAGE_SIZE, can stop an array from
working.

For RAID10 and RAID4/5/6, this is non-trivial to fix as the resync
process works on whole pages at a time, and assumes them to be wholly
within a stripe.  For other raid personalities, this restriction is
not needed at all and can be dropped.

So remove the test on chunk_size from common can, and add it in just
the places where it is needed: raid10 and raid4/5/6.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -3520,17 +3520,12 @@ static int do_md_run(mddev_t * mddev)
 			return -EINVAL;
 		}
 		/*
-		 * chunk-size has to be a power of 2 and multiples of PAGE_SIZE
+		 * chunk-size has to be a power of 2
 		 */
 		if ( (1 << ffz(~chunk_size)) != chunk_size) {
 			printk(KERN_ERR "chunk_size of %d not valid\n", chunk_size);
 			return -EINVAL;
 		}
-		if (chunk_size < PAGE_SIZE) {
-			printk(KERN_ERR "too small chunk_size: %d < %ld\n",
-				chunk_size, PAGE_SIZE);
-			return -EINVAL;
-		}
 
 		/* devices must have minimum size of one chunk */
 		rdev_for_each(rdev, tmp, mddev) {