growfs: do not error if filesystem is already requested size

For some cloud/virtualization use cases it can be convenient to grow the
filesystem on boot any time the disk/partition happens to be larger, but
not fail if it remains the same size.

Continue to emit a message if we have no action to take, but exit with
status 0 if the size remains the same.

Reviewed by: trasz
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32856

(cherry picked from commit 3f9acedb020b4bcac850eee8a60c75e0880d3e3f)