Lock tmpfs node tn_status updates done under the shared vnode lock.

If tmpfs vnode is only shared locked, tn_status field still needs
updates to note the access time modification. Use the same locking
scheme as for UFS, protect tn_status with the node interlock + shared
vnode lock.

Fix nearby style.

Noted and reviewed by: mjg
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week