Convert ksyms(4) to use an OBJT_PHYS object.

The pages stored in the ksyms object are not pageable. Moreover, this
obviates the need to set OBJ_NOSPLIT.

Reviewed by: alc, kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22229