ctl ramdisk: Free compare buffer after a compare I/O request.

For a compare request, the ramdisk backend allocates a temporary
buffer to hold the I/O data and then compares it against the LUN's
pages in ctl_backend_ramdisk_cmp after the data has been filled.
However, the tempory buffer was leaked when after the comparison was
complete. Fix this by freeing the buffer after the comparison.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D34316

(cherry picked from commit bd6e8729d6f6d42102b0359b56b7e3c89edd0ec9)