Use BUG_ON() instead of an if condition followed by BUG().

The semantic patch that makes this change is available in
scripts/coccinelle/misc/bugon.cocci.
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
CC: Pratyush Anand <pratyush.anand@gmail.com>