CVE-2026-74365: nvdimm/btt: Handle preemption in BTT lane acquisition

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

nvdimm/btt: Handle preemption in BTT lane acquisition

BTT lanes serialize access to per-lane metadata and workspace state during BTT I/O. The btt-check unit test reports data mismatches during BTT writes due to a race in lane acquisition that can lead to silent data corruption.

The existing lane model uses a spinlock together with a per-CPU recursion count. That recursion model stopped being valid after BTT lanes became preemptible: another task can run on the same CPU, observe a non-zero recursion count, bypass locking, and use the same lane concurrently.

BTT lanes are also held across arenawritebytes() calls. That path reaches nsiorwbytes(), which flushes writes with nvdimmflush(). Some provider flush callbacks can sleep, making a spinlock the wrong primitive for the lane lifetime.

Replace the spinlock-based recursion model with a dynamically allocated per-lane mutex array and take the lane lock unconditionally.

Add mightsleep() to catch any future atomic-context caller.

Found with the ndctl unit test btt-check.sh.