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

[jhb]

Summary

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.

Sponsored by: Chelsio Communications

[jhb]

I haven't figured out a good way to test this (don't see a way to easily generate COMPARE WRITE or VERIFY commands to daX), just noticed this while investigating a possible memory leak reported by Chelsio's QA folks.

[mav]

Looks good to me.

There are sg_compare_and_write and sg_verify in sg3_utils port.

[jhb]

Thanks, I tested with sg_verify and sg_compare_and_write.