linux(4): Bump osrelease to 5.15.0

Linux kernel version 5.15 named Trick or Treat is a 22nd LTS release.

Reviewed by: trasz, emaste
Differential Revision: https://reviews.freebsd.org/D39649
MFC after: 1 month

(cherry picked from commit 0ef30b5d6b75596e087a86c3cb01a0b2c611e7e6)