md/raid1/10: add missed blk plug

[ Upstream commit 18022a1bd3709b74ca31ef0b28fccd52bcd6c504 ]

flush_pending_writes isn't always called with block plug, so add it, and plug
works in nested way.

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>