ion: Make user_ion_handle_put_nolock() a void function

Clang warns:

  drivers/staging/android/ion/ion-ioctl.c:71:6: warning: variable 'ret' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
          if (--handle->user_ref_count == 0)
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/staging/android/ion/ion-ioctl.c:74:9: note: uninitialized use occurs here
          return ret;
                 ^~~
  drivers/staging/android/ion/ion-ioctl.c:71:2: note: remove the 'if' if its condition is always true
          if (--handle->user_ref_count == 0)
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/staging/android/ion/ion-ioctl.c:69:9: note: initialize the variable 'ret' to silence this warning
          int ret;
                 ^
                  = 0
  1 warning generated.

The return value of user_ion_handle_put_nolock() is not checked in its
one call site in user_ion_free_nolock() so just make
user_ion_handle_put_nolock() return void to remove the warning.

Fixes: a8200613 ("ion: Protect kref from userspace manipulation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>