ip_mroute: allow MRT_DEL_MFC even if there are pending upcalls

In del_mfc() we try to find the forwarding entry, but that fails to find
the entry if mfc_stall_ring is not empty.

We should find the entry anyway, and destroy it (and any pending
messages) on delete.

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43031