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.

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

(cherry picked from commit e8449c0e0fcb8a3eb5872cbee5c3dde4b05a5f50)
(cherry picked from commit a23bd71deaad478cbe41ae756b96ea7a32537311)