MD: Memory leak when flush bio size is zero

[ Upstream commit af9b926de9c5986ab009e64917de87c9758bab10 ]

flush_pool is leaked when flush bio size is zero

Fixes: 5a409b4f ("MD: fix lock contention for flush bios")
Signed-off-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>