fix #4470: pct fstrim: ignore bind or read-only mountpoints

fix #4470: pct fstrim: ignore bind or read-only mountpoints

Currently, `pct fstrim` will run `fstrim` on all mountpoints
of the container, including bind and read-only mountpoints.

However, trimming a bind mountpoint might trim a host
filesystem, which users may not expect. Also, trimming can
be considered a write operation, which users may not expect
to be carried out on a read-only mountpoint.

Hence, exclude bind mointpoints and read-only mountpoints
from trimming.

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>