mount: rename mount_state_active() → MOUNT_STATE_WITH_PROCESS()

mount: rename mount_state_active() → MOUNT_STATE_WITH_PROCESS()

The function returns true for all states that have a control process
running, and each time we call it that's what we want to know, hence
let's rename it accordingly. Moreover, the more generic unit states have
an ACTIVE state, and it is defined quite differently from the set of
states this function returns true for, hence let's avoid confusion and
not reuse the word "ACTIVE" here in a different context.

Finally, let's uppercase this, since in most ways it's pretty much
identical to a macro