rmlock(9): add an RM_DUPOK flag

Allows for duplicate locks to be acquired without witness complaining.
Similar flags exists already for rwlock(9) and sx(9).

Reviewed by: markj
MFC after: 3 days
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
NetApp PR: 52
Differential Revision: https://reviews.freebsd.org/D29683n