zvol state locking: refinements for scalability

References:
openzfs#2479
openzfs@79c76d5 (Change KM_PUSHPAGE -> KM_SLEEP)

@@ __zvol_create_minor(const char *name)

    doi = kmem_alloc(sizeof (dmu_object_info_t), KM_PUSHPAGE);

    [replaced with]

    doi = kmem_alloc(sizeof (dmu_object_info_t), KM_SLEEP);

    Caused by

    79c76d5
    Change KM_PUSHPAGE -> KM_SLEEP
    (upstream sync).

Fixing cstyle errors:

space or tab at end of line

indent by spaces instead of tabs

missing space between keyword and paren

unparenthesized return expression