um: Allocate vdso page pointer statically

Instead of dynamically allocating the pointer to the vdso page during
boot, we can just allocate it statically. Doing so will reduce error
handling and make the code slightly more readable.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250212045756.164977-1-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>