Change ISC_R_EXISTS to ISC_R_SUCCESS in dns/rbtdb.c:findnodeintree()
In the previous refactoring, the findnodeintree() function could return ISC_R_EXISTS (from dns_db_addnode() call) instead of ISC_R_SUCCESS leading to node being attached, but never detached.
Change the ISC_R_EXISTS result code returned from dns_rbt_addnode() to the ISC_R_SUCCESS in the findnodeintree() function (called internally by dns_db_findnode() and dns_db_findnsec3node()).
Closes #3671 (closed)