postgresql: fix fails to start on mounted data directories

postgresql: fix fails to start on mounted data directories

This commit resolves a bug where the PostgreSQL service failed to start
during the boot process when its data directory ($pgdata) was on a
partition mounted via `/etc/fstab`.
Because the filesystem is already mounted, no hotplug event is ever
triggered, and the PostgreSQL service mistakenly waits for an event
that will never happen.
This fix adds a check to ensure the script does not skip starting the
service if the data directory is on a mount point.

Signed-off-by: Peiying Huang <[email protected]>