mgr/dashboard: cephfs subvolume creation form

[nizamial09]

Fixes: https://tracker.ceph.com/issues/62345

scrnli_8_4_2023_4-09-13.PM.mp4

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  • References tracker ticket
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[pereman2]

small typo but lgtm

[cloudbehl]

Amazing work! Thanks Nizam!

Tested this and LGTM.