iio: buffer: re-introduce bitmap_zalloc() for trialmask

Commit 3862828a ("iio: buffer: Switch to bitmap_zalloc()") introduced
bitmap_alloc(), but commit 20ea39ef ("iio: Fix scan mask selection")
reverted it.

This change adds it back. The only difference is that it's adding
bitmap_zalloc(). There might be some changes later that would require
initializing it to zero. In any case, now it's already zero-ing the
trialmask.

Appears to have been the result of merge conflict resolution rather
than an intentional revert.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>