[XFS] Fix a potential NULL pointer deref in XFS on failed mount.

If we fail to open the the log device buftarg, we can fall through to
error handling code that fails to check for a NULL log device buftarg
before calling xfs_free_buftarg().

This patch fixes the issue by checking mp->m_logdev_targp against NULL in
xfs_unmountfs_close() and doing the proper xfs_blkdev_put(logdev); and
xfs_blkdev_put(rtdev); on (!mp->m_rtdev_targp) in xfs_mount().

Discovered by the Coverity checker.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29328a
Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>

fs/xfs/xfs_mount.c

@@ -1262,7 +1262,7 @@ xfs_unmountfs(xfs_mount_t *mp, struct cred *cr)
 void
 xfs_unmountfs_close(xfs_mount_t *mp, struct cred *cr)
 {
-	if (mp->m_logdev_targp != mp->m_ddev_targp)
+	if (mp->m_logdev_targp && mp->m_logdev_targp != mp->m_ddev_targp)
 		xfs_free_buftarg(mp->m_logdev_targp, 1);
 	if (mp->m_rtdev_targp)
 		xfs_free_buftarg(mp->m_rtdev_targp, 1);

fs/xfs/xfs_vfsops.c

@@ -480,13 +480,19 @@ xfs_mount(
 	}
 	if (rtdev) {
 		mp->m_rtdev_targp = xfs_alloc_buftarg(rtdev, 1);
-		if (!mp->m_rtdev_targp)
+		if (!mp->m_rtdev_targp) {
+			xfs_blkdev_put(logdev);
+			xfs_blkdev_put(rtdev);
 			goto error0;
 		}
+	}
 	mp->m_logdev_targp = (logdev && logdev != ddev) ?
 				xfs_alloc_buftarg(logdev, 1) : mp->m_ddev_targp;
-	if (!mp->m_logdev_targp)
+	if (!mp->m_logdev_targp) {
+		xfs_blkdev_put(logdev);
+		xfs_blkdev_put(rtdev);
 		goto error0;
+	}
 
 	/*
 	 * Setup flags based on mount(2) options and then the superblock