fuse: extend init flags

FUSE_INIT flags are close to running out, so add another 32bits worth of
space.

Add FUSE_INIT_EXT flag to the old flags field in fuse_init_in.  If this
flag is set, then fuse_init_in is extended by 48bytes, in which a flags_hi
field is allocated to contain the high 32bits of the flags.

A flags_hi field is also added to fuse_init_out, allocated out of the
remaining unused fields.

Known userspace implementations of the fuse protocol have been checked to
accept the extended FUSE_INIT request, but this might cause problems with
other implementations.  If that happens to be the case, the protocol
negotiation will have to be extended with an extra initialization request
roundtrip.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>