tmpfs: update changed/modified timestamps for truncates that do not change size
While there, move all error checks into the common place at the start,
and eliminate the 'out' label.
PR: 268528
Analyzed and tested by: Mark Millard <marklmi@yahoo.com>
Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37866