rust: move all file operation callbacks into FileOperationsVtable

This is a pure refactor with no functional change intended.

Since the callbacks are only used to build the table, they logically
belong together; it also obviates the need to repeat the generic
parameter and bound `T: FileOperations` in all callbacks.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>

Author: wedsonaf