Fix switch-bool warning

Fix switch-bool warning

As of gcc version 5.1.1 a new warning has been added to detect the
use of a boolean in a switch statement (-Wswitch-bool).  Resolve the
warning by explicitly casting the value to an integer type.

  zfs-0.6.4/module/zfs/zvol.c: In function 'zvol_request':
  error: switch condition has boolean value [-Werror=switch-bool]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>