linuxkpi: Move cdev_alloc into .c file

Move cdev_alloc into linux_compat.c since it encodes the size of struct
linux_cdev into the client modules otherwise.

Sponsored by: Netflix
Reviewed by: hselasky, emaste
Differential Revision: https://reviews.freebsd.org/D34771

(cherry picked from commit aca0bcbca344b39f2a448e8c1c0d80bac8c0d6de)