memdesc: Retire MEMDESC_BIO.

[jhb]

Summary

Instead, change memdesc_bio to examine the bio and return a memdesc of
a more generic type describing the data buffer.

[markj]

sys/kern/subr_msan.c

550

This function is used externally, currently just by vtblk_bio_done(). In fact that might no longer be necessary now that virtio-blk uses busdma, but we should keep it anyway IMO; kmsan_mark*() is intended to be called externally.

572

Same here, used by GEOM.

[jhb]

sys/kern/subr_msan.c

550

I will leave the functions, though arguably they need updating for unmapped bios at some point.

[markj]

sys/kern/subr_msan.c

550

That case is handled implicitly: unmapped BIOs have bio_data == unmapped_buf. In general kmsan_mark()/kmsan_check() are simply no-ops if the address passed in doesn't lie within the kernel map, so even bio_data == NULL would be fine.