mlx4core: Fix a memory leak when deleting slave's resources

mlx4_delete_all_resources_for_slave() in the resource tracker should free
all memory allocated for a slave. While releasing memory of fs_rule,
it misses releasing memory of fs_rule->mirr_mbox.

Linux commit:
461d5f1b59490ce0096dfda45e10038c122a7892

PR: 264249
MFC after: 1 week
Sponsored by: NVIDIA Networking