fusefs: respect the server's FUSE_SETXATTR_EXT flag

FUSE protocol 7.33 extended the FUSE_SETXATTR request format. But the
extension is optional. The server must opt-in by setting the
FUSE_SETXATTR_IN flag during FUSE_INIT. We were wrongly using the
extended format for any server using protocol 7.33 or later.

PR: 290547
Co-authored-by: CismonX <admin@cismon.net>
Fixes: d5e3cf41e89 ("fusefs: Upgrade FUSE protocol to version 7.33")
MFC after: 3 days