s390/s390dbf: add debug_level_enabled() function

Add the debug_level_enabled() function to check if debug events for
a particular level would be logged.  This might help to save cycles
for debug events that require additional information collection.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>