Annotate 'rebooting' with __read_mostly

While here, put such annotation after the variable for 'dumping', since
it concerns the variable and not the type.

Reviewed by: markj
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43570

(cherry picked from commit 12d6a032dff10fad72f7d3f1bb5cb4c53b9c4284)

Approved by: markj (mentor)