mm: fix refcounting in swapon

Grab a reference to bdev before calling blkdev_get(), which expects
the refcount to be already incremented and either returns success or
decrements the refcount and returns an error.

The bug was introduced by e525fd89 (block: make blkdev_get/put()
handle exclusive access), which didn't take into account this behavior
of blkdev_get().
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -1940,7 +1940,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
 	error = -EINVAL;
 	if (S_ISBLK(inode->i_mode)) {
-		bdev = I_BDEV(inode);
+		bdev = bdgrab(I_BDEV(inode));
 		error = blkdev_get(bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL,
 				   sys_swapon);
 		if (error < 0) {