Bug fix in qat_compress.c when compressed size is < 4KB

Bug fix in qat_compress.c when compressed size is < 4KB

When the 128KB block is compressed to less than 4KB, the pointer
to the Footer is not in the end of the compressed buffer, that's
because the Header offset was added twice for this case. So there
is a gap between the Footer and the compressed buffer.
1. Always compute the Footer pointer address from the start of the
last page.
2. Remove the un-used workaroud code which has been verified fixed
with the latest driver and this fix.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Weigang Li <weigang.li@intel.com>
Closes #6827