fuse: use exclusive lock when FUSE_I_CACHE_IO_MODE is set

This may be a typo. The comment has said shared locks are
not allowed when this bit is set. If using shared lock, the
wait in `fuse_file_cached_io_open` may be forever.

Fixes: 205c1d80 ("fuse: allow parallel dio writes with FUSE_DIRECT_IO_ALLOW_MMAP")
CC: stable@vger.kernel.org # v6.9
Signed-off-by: yangyun <yangyun50@huawei.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>