mgr/dashboard: fix cephfs name validation

[nizamial09]

allow volume name to start with dot (.)

Fixes: https://tracker.ceph.com/issues/65143

Contribution Guidelines

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Checklist

• Tracker (select at least one)
  • References tracker ticket
  • Very recent bug; references commit where it was introduced
  • New feature (ticket optional)
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  • Affects Dashboard, opened tracker ticket
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[nizamial09]

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[nizamial09]

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