Add "bitmap" to allowed command-line values

Add "bitmap" to allowed command-line values

Currently, the sysfs interface allows bitmap setup only when volume
is in an inactive state.
For external metadata to add bitmap to existing volume instead of
GROW, the UPDATE operation can be done.
The patch adds a "bitmap" argument to the allowed values for UPDATE.

Signed-off-by: Jakub Radtke <jakub.radtke@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>