vnode alloc bitnumber returns bitnumber

vnode alloc bitnumber returns bitnumber

fix function return types to match a bitnumber (int)
and comment the function appropriately

Change-Id: I9542c02b1aa7aacdd0596675992bb1e8a1708572
Reviewed-on: http://gerrit.openafs.org/1941
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>