rgw: avoid string_view to temporary in RGWBulkUploadOp

rgw: avoid string_view to temporary in RGWBulkUploadOp

the `else` block below constructs a temporary std::string that destructs
at the end of the statement, leaving `filename` as a dangling view:
```
filename = file_prefix + std::string(header->get_filename());
```
store a copy of the `std::string` instead

Fixes: https://tracker.ceph.com/issues/57326
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit e9c2ac02f1f59714677e3e1b984367df3c0ec166)