[PATCH] Fix bd_claim() error code.

Problem: In some circumstances, bd_claim() is returning the wrong error
code.

If we try to swapon an unused block device that isn't swap formatted, we
get -EINVAL.  But if that same block device is already mounted, we instead
get -EBUSY, even though it still isn't a valid swap device.

This issue came up on the busybox list trying to get the error message
from "swapon -a" right.  If a swap device is already enabled, we get -EBUSY,
and we shouldn't report this as an error.  But we can't distinguish the two
-EBUSY conditions, which are very different errors.

In the code, bd_claim() returns either 0 or -EBUSY, but in this case busy
means "somebody other than sys_swapon has already claimed this", and
_that_ means this block device can't be a valid swap device.  So return
-EINVAL there.
Signed-off-by: NRob Landley <rob@landley.net>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/swapfile.c

@@ -1381,6 +1381,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 		error = bd_claim(bdev, sys_swapon);
 		if (error < 0) {
 			bdev = NULL;
+			error = -EINVAL;
 			goto bad_swap;
 		}
 		p->old_block_size = block_size(bdev);