drivers/md/md-bitmap: check the return value of md_bitmap_get_counter()

[ Upstream commit 3bd548e5b819b8c0f2c9085de775c5c7bff9052f ]

Check the return value of md_bitmap_get_counter() in case it returns
NULL pointer, which will result in a null pointer dereference.

v2: update the check to include other dereference

Signed-off-by: Li Zhong <floridsleeves@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>