linux-user: Add support for FDFLUSH ioctl

linux-user: Add support for FDFLUSH ioctl

FDFLUSH is used for flushing buffers of floppy drives. Support in
QEMU is needed because some of Debian packages use this ioctl while
running post-build tests. One such example is 'tar' package.

Signed-off-by: Yunqiang Su <ysu@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <1567601968-26946-5-git-send-email-aleksandar.markovic@rt-rk.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>