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

sbin/growfs/growfs.c

@@ -1503,7 +1503,10 @@ main(int argc, char **argv)
 		humanize_number(newsizebuf, sizeof(newsizebuf), size,
 		    "B", HN_AUTOSCALE, HN_B | HN_NOSPACE | HN_DECIMAL);
 
-		errx(1, "requested size %s is not larger than the current "
+		if (size == (uint64_t)(osblock.fs_size * osblock.fs_fsize))
+			errx(0, "requested size %s is equal to the current "
+			    "filesystem size %s", newsizebuf, oldsizebuf);
+		errx(1, "requested size %s is smaller than the current "
 		   "filesystem size %s", newsizebuf, oldsizebuf);
 	}