Return -EINVAL if read() or write() is not supported by the queue. This
makes it possible to provide both vb2_fop_read and vb2_fop_write in a
struct v4l2_file_operations since the vb2_fop_* function will check if
the file operation is allowed.

A similar check exists in __vb2_init_fileio() which is called from
__vb2_perform_fileio(), but that check is only done if no file I/O is
active. So the sequence of read() followed by write() would be allowed,
which is obviously a bug.

In addition, vb2_fop_write/read should always return -EINVAL if the
operation is not allowed, and by putting the check in the lower levels
of the code it is possible that other error codes are returned (EBUSY
or ERESTARTSYS).

All these issues are avoided by just doing a quick explicit check.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Acked-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>