The two functions are identical, so there's no need to keep two copies
of the same code around. Alias block_op_close() to block_op_flush()
and drop standalone definition for the former.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>