mlx5_core: fix deadlock when using RXTLS

If removing a node of type FS_TYPE_FLOW_DEST we lock the flow group too
late. This can lead to a deadlock with fs_add_dst_fg().

PR: 274715
MFC after: 1 week
Reviewed by: kib
Tested by: mm
Differential Revision: https://reviews.freebsd.org/D42368