[media] videobuf2: avoid memory leak on errors

As reported by smatch:
	drivers/media/v4l2-core/videobuf2-core.c:2415 __vb2_init_fileio() warn: possible memory leak of 'fileio'

While here, avoid the usage of sizeof(struct foo_struct).

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>