btrfs: fix varargs in __btrfs_std_error

__btrfs_std_error didn't always properly call va_end,
and might call va_start even if fmt was NULL.

Move all the varargs handling into the block where we
have fmt.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>