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
https://github.com/libfuse/libfuse/pull/662



Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Fixes: 53db2893 ("fuse: extend init flags")
Cc: <stable@vger.kernel.org> # v5.17
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>