MFC growfs script change from August 2021:

• Fix the growfs rc script to cope with diskid labels.
• Fix a warning in growfs. gpart commit is supposed to be called on disk device.
• Silence a gpart commit warning in growfs.

Submitted by: loos
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D31587
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 045001075ed18295288176211456dde3df7faf46)