fuse: Apply flags2 only when userspace set the FUSE_INIT_EXT

This is just a safety precaution to avoid checking flags on memory that was
initialized on the user space side.  libfuse zeroes struct fuse_init_out
outarg, but this is not guranteed to be done in all implementations.
Better is to act on flags and to only apply flags2 when FUSE_INIT_EXT is
set.

There is a risk with this change, though - it might break existing user
space libraries, which are already using flags2 without setting
FUSE_INIT_EXT.

The corresponding libfuse patch is here
libfuse/libfuse#662

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Fixes: 53db289 ("fuse: extend init flags")
Cc: <stable@vger.kernel.org> # v5.17
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: bsbernd

fs/fuse/inode.c

@@ -1134,7 +1134,10 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 		process_init_limits(fc, arg);
 
 		if (arg->minor >= 6) {
-			u64 flags = arg->flags | (u64) arg->flags2 << 32;
+			u64 flags = arg->flags;
+
+			if (flags & FUSE_INIT_EXT)
+				flags |= (u64) arg->flags2 << 32;
 
 			ra_pages = arg->max_readahead / PAGE_SIZE;
 			if (flags & FUSE_ASYNC_READ)