tmpfs: restore atime updates for reads from page cache.

Split TMPFS_NODE_ACCCESSED bit into dedicated byte that can be updated
atomically without locks or (locked) atomics.

tn_update_getattr() change also contains unrelated bug fix.

Reported by: lwhsu
PR: 249362
Reviewed by: markj (previous version)
Discussed with: mjg
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D26451