sys/generated: fix showprio reports NoExist error #17536

Job	Run time
build	12s
aux	2m 25s
race_dashboard	11s
old	2m 27s
arch (presubmit_arch_netbsd)	2m 27s
arch (presubmit_arch_freebsd)	2m 27s
dashboard	8s
arch (presubmit_arch_darwin)	2m 27s
race	1s
arch (presubmit_arch_windows)	2m 28s
gvisor	2m 26s
arch (presubmit_arch_linux)	2m 27s
arch (presubmit_arch_openbsd)	2m 27s
arch (presubmit_arch_executor)	2m 27s
	25m 0s

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