Fix error message for case of kernel prior to 2.6.30
Fix error message for case of kernel prior to 2.6.30

The error message was falsely claiming that a debugfs directory
didn't exist, when it was supposed to report that the ringbuffer
file did not exist within that directory.

tiagovignatti/intel-gpu-tools,erikarn/intel-gpu-tools,rib/intel-gpu-tools,rib/intel-gpu-tools,dlespiau/checkmate-test-igt,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools,chenxianqin/intel-gpu-tools,yipdw/intel-gpu-tools,dlespiau/intel-gpu-tools,dlespiau/checkmate-test-igt,yipdw/intel-gpu-tools,tiagovignatti/intel-gpu-tools,mv0/intel-gpu-tools,yipdw/intel-gpu-tools,rib/intel-gpu-tools,tiagovignatti/intel-gpu-tools,erikarn/intel-gpu-tools,dlespiau/intel-gpu-tools,ChrisCummins/intel-gpu-tools,erikarn/intel-gpu-tools,dlespiau/checkmate-test-igt,erikarn/intel-gpu-tools,chenxianqin/intel-gpu-tools,ChrisCummins/intel-gpu-tools,rib/intel-gpu-tools,tiagovignatti/intel-gpu-tools,dlespiau/checkmate-test-igt,mv0/intel-gpu-tools,yipdw/intel-gpu-tools,chenxianqin/intel-gpu-tools,ChrisCummins/intel-gpu-tools,mv0/intel-gpu-tools,ChrisCummins/intel-gpu-tools,dlespiau/intel-gpu-tools