dax: Inline dax_insert_mapping() into the callsite

dax_insert_mapping() has only one callsite and we will need to further
fine tune what it does for synchronous faults. Just inline it into the
callsite so that we don't have to pass awkward bools around.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>