selinux: Avoid dynamic memory allocation for small context buffers

selinux: Avoid dynamic memory allocation for small context buffers

Most context buffers are rather small and can fit on the stack,
eliminating the need to allocate them dynamically. Reserve a 4 KiB
stack buffer for this purpose to avoid the overhead of dynamic
memory allocation.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>