kern_malloc: remove '.' and '\n' from KASSERT/panic

KASSERT and panic calls should not have a '.' at the end of the message
and do not need '\n'. Remove these.

No functional changes.

Reported by: markj (during review of D45812)
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: markj, kib
Differential Revision: https://reviews.freebsd.org/D46093