scx_fair: use cast_mask() from common.bpf.h #578

Job	Run time
lint	17s
build-kernel	28s
pages	0s
rust-test-core (scx_loader)	3m 24s
rust-test-schedulers (scx_bpfland)	4m 54s
rust-test-core (scx_rustland_core)	4m 59s
integration-test (scx_bpfland)	3m 31s
rust-test-schedulers (scx_lavd)	4m 55s
rust-test-core (scx_stats)	2m 33s
integration-test (scx_lavd)	4m 2s
rust-test-schedulers (scx_layered)	5m 1s
rust-test-core (scx_utils)	5m 4s
integration-test (scx_layered)	3m 53s
rust-test-schedulers (scx_rlfifo)	5m 10s
integration-test (scx_rlfifo)	3m 8s
rust-test-schedulers (scx_rustland)	5m 37s
integration-test (scx_rustland)	5m 0s
rust-test-schedulers (scx_rusty)	6m 8s
integration-test (scx_rusty)	4m 46s
	1h 12m 50s

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