CVE-2026-74370: liveupdate: fix TOCTOU race in luo_session_retrieve()

In the Linux kernel, the following vulnerability has been resolved:

liveupdate: fix TOCTOU race in luosessionretrieve()

Extend the scope of the rwsemread lock in luosessionretrieve() to overlap with the acquisition of the session mutex. This prevents a concurrent thread from releasing and freeing the session between the lookup and the mutex lock.