md(4): Add dummy support of the BIO_FLUSH command for malloc and swap
backend.
PR: 260200
Reported by: editor@callfortesting.org
Reviewed by: vmaffione (mentor), markj
Approved by: vmaffione (mentor), markj
Differential Revision: https://reviews.freebsd.org/D34260
(cherry picked from commit cb28dfb27d12f1cbd7831f370c01493c01d74b10)