fuse: don't allow signals to interrupt getdents copying

When getting the directory contents, the entries are first fetched to a
kernel buffer, then they are copied to userspace with dir_emit().  This
second phase is non-blocking as long as the userspace buffer is not paged
out, making it interruptible makes zero sense.

Overload d_type as flags, since it only uses 4 bits from 32.

Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>