contrib: dracut: zfs-snapshot-bootfs: exit status fix

When the zfs-snapshot-bootfs service attempts to create a snapshot
that already exists, the exit status of the command is non-zero and
the service reports failed to the systemd service manager. This is a
common occurrence if bootfs.snapshot is left set on the kernel command
line and it should not be considered a failure.

This service was originally set to ignore this error by prefixing
the command with - on the ExecStart line, but the leading - appears
to have been dropped in #13359.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gregory Bartholomew <gregory.lee.bartholomew@gmail.com>
Closes #13769