md/linear: shutup lockdep warnning

[ Upstream commit d939cdfd ]

Commit 03a9e24e(md linear: fix a race between linear_add() and
linear_congested()) introduces the warnning.

Acked-by: Coly Li <colyli@suse.de>
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>