Rename linux_set_upcall_kse() to linux_set_upcall().

This matches the rename of cpu_set_upcall_kse() in
5c2cf818454375536fda522ba83cf67c50929e6b.

Reviewed by: kib, emaste
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D29295