um: Calculate stub data address relative to stub code

um: Calculate stub data address relative to stub code

Instead of using the current stack pointer, we can also use the current
instruction to calculate where the stub data is. With this the stub data
only needs to be aligned to a full page boundary.

Changing this has the advantage that we do not have a hole in the memory
space above the stub data (which would need to be explicitly cleared).

Another motivation to do this is that with the planned addition of a
SECCOMP based userspace the stack pointer may not be fully trustworthy.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20240919124511.282088-7-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>