mount-util: add name_to_handle_at_loop() wrapper around name_to_handle_at()

mount-util: add name_to_handle_at_loop() wrapper around name_to_handle_at()

As it turns out MAX_HANDLE_SZ is a lie, the handle buffer we pass into
name_to_handle_at() might need to be larger than MAX_HANDLE_SZ, and we
thus need to invoke name_to_handle_at() in a loop, growing the buffer as
needed.

This adds a new wrapper name_to_handle_at_loop() around
name_to_handle_at() that does the necessary looping, and ports over all
users.

Fixes: #7082