Update kernel version to 5.15 in tst-mman-consts.py

This patch updates the kernel version in the test tst-mman-consts.py
to 5.15.  (There are no new MAP_* constants covered by this test in
5.15 that need any other header changes.)

Tested with build-many-glibcs.py.

(cherry picked from commit 5c3ece45)