Make GEOM DISK to account also BIO_FLUSH operations.

(cherry picked from commit f5e9ad20708e345e9da1d86a18fd03abce1c0608)
Add -o option to gstat to display "other" operatins (e.g. BIO_FLUSH).

MFC after:	1 week

(cherry picked from commit a818dea3784b972aeb788d779d0cfdd392f3506b)
(cherry picked from commit bd5d8be98853b2a135c414071e57278304d79c7f)