fs: extract out validatePath function

PR-URL: #17682
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>