tcrypt: new debugfs interface

Tcrypto is a kernel module which can be used to trigger speed tests on
different parts of the crypto API. To trigger a test, users has to load the module
using specific module parameters, e.g.:

 # modprobe tcrypt mode=404 sec=5

All tcrypt tests are implemented in the _init function, their output is sent to
the printk buffer, and once the test is over _init always return a failure.

This patch modifies tcrypt in order to exposes a debugfs user space interface,
which can be used to trigger tcrypt tests and to get test output.

Three files are available in the tcrypt debugfs directory:

 - command: this file can be use to control which test should be run simply
            writing the test name to it. it also supports clearing the 'output'
            buffer and listing all available tests

 - output: holds a ring buffer with the output of the tests

 - second: this file can be used to control the number of seconds that a
           tests should be run consecutively. If zero it uses cpu cycles