server: Reply FUSE_INIT_EXT on FUSE_INIT

Currently, the kernel applies InitOut.flags2 without checking whether FUSE_INIT_EXT set in InitOut.flags, but this could change in the future.

See: https://lore.kernel.org/all/c63d9fdb-a980-c151-8a14-d62706fd17f8@linux.alibaba.com/T/