tmpfs: Move partial page invalidation to a separate helper

The partial page invalidation code is factored out to be a separate
helper from tmpfs_reg_resize().

Sponsored by: The FreeBSD Foundation
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31683

(cherry picked from commit 399be91098adb23aa27ca1228b81a3ad67e8bba2)