md/md-multipath: guard against a possible NULL dereference

mempool_alloc might fail to allocate a slot, in which case we will end
up dereferencing a NULL mp_bh pointer.

Found by Linux Verification Center (linuxtesting.org) with the SVACE
static analysis tool.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>

drivers/md/md-multipath.c

@@ -108,6 +108,8 @@ static bool multipath_make_request(struct mddev *mddev, struct bio * bio)
 		return true;
 
 	mp_bh = mempool_alloc(&conf->pool, GFP_NOIO);
+	if (!map_bh)
+		return false;
 
 	mp_bh->master_bio = bio;
 	mp_bh->mddev = mddev;