ctl: Add CTL_IO_ASSERT wrapper macro

Currently, this pattern is commonly used to assert that a union ctl_io
is a SCSI request. In the future it will be used to assert other
types.

Suggested by: imp
Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44844

(cherry picked from commit ac7a514e20f394ffed0126c0c356aa9107821880)