mon: AuthMonitor: don't return global_id right away if we're increasing it

This only happens on the Leader and leads to duplicate global_ids.

Fixes: #4285

Signed-off-by: Joao Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
mon: simplify assign_global_id()

Simplify the logic a bit so it is easier to follow.

Small behavior change: we will successfully allocate and return a gid that
== the max when we can't bump it.

Signed-off-by: Sage Weil <sage@inktank.com>