[ Upstream commit af9b926d ]

flush_pool is leaked when flush bio size is zero

Fixes: 5a409b4f ("MD: fix lock contention for flush bios")
Signed-off-by: NDavid Jeffery <djeffery@redhat.com>
Signed-off-by: NXiao Ni <xni@redhat.com>
Signed-off-by: NShaohua Li <shli@fb.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>