bitops: protect variables in bit_clear_unless() macro

Unprotected naming of local variables within bit_clear_unless() can easily
lead to using the wrong scope.

Noticed this by code review after having hit this issue in set_mask_bits()

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 85ad1d13 ("md: set MD_CHANGE_PENDING in a atomic region")
Cc: Guoqing Jiang <gqjiang@suse.com>