x86/RAS/mce_amd_inj: Remove debugfs dir recursively on exit

Simplify exit_mce_inject() by using debugfs_remove_recursive() and do
away with the noodling over the dentry elements.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160926083152.30848-3-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>