kernel: Disable few nonnull attributes.

kernel: Disable few nonnull attributes.

To avoid -Wnonnull-compare, similarly as in:
0f037c78525bfb7df49de8338826e15137859b1d

devfs_core.c:   devfs_find_device_by_name()
			1st arg checked (NULL == return)
kern_slaballoc.c: kfree()
			1st arg checked (NULL == panic)
			2nd arg not checked
kern_synch.c:   tsleep()
			1st arg checked (NULL == KASSERT)
subr_prf.c:	kvcprintf()
			1st arg checked (NULL == "(fmt null)")
subr_sbuf.c:	sbuf_vprintf()
			2nd arg checked (NULL == KASSERT)