bio: make _bio_cflags match bio_cflags

While none of the in-tree geoms that modify the bio_cflags use the top
half of the word, were they to do this in the future, we'd hit false
positives for DIAGNOSTIC kernels. Make both uint16_t.

MFC After: 1 week
Sponsored by: Netflix

(cherry picked from commit 077ab5bd43aaeb955d84c501ddddc7533f0a06b9)