habanalabs: fix debugfs code

This fixes multiple things in the habanalabs debugfs code, in particular:

 - mmu_write() was unnecessarily verbose, copying around between multiple
   buffers
 - mmu_write() could write a user-specified, unbounded amount of userspace
   memory into a kernel buffer (out-of-bounds write)
 - multiple debugfs read handlers ignored the user-supplied count,
   potentially corrupting out-of-bounds userspace data
 - hl_device_read() was unnecessarily verbose
 - hl_device_write() could read uninitialized stack memory
 - multiple debugfs read handlers copied terminating null characters to
   userspace

Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Cc: stable@vger.kernel.org