linuxkpi: Define `might_alloc()` as a no-op

[dumbbell]

Summary

This is part of the update of the DRM drivers to Linux 5.14.

[manu]

What does it do in Linux ?

[bz]

sys/compat/linuxkpi/common/include/linux/mm.h

352

If it stays like this please add `__unused`

[dumbbell]

In D38146#866956, @manu wrote:

What does it do in Linux ?

It is some kind of code annotation to let tools to know which places may allocate memory and perhaps sleep too:

/**
 * might_alloc - Mark possible allocation sites
 * @gfp_mask: gfp_t flags that would be used to allocate
 *
 * Similar to might_sleep() and other annotations, this can be used in functions
 * that might allocate, but often don't. Compiles to nothing without
 * CONFIG_LOCKDEP. Includes a conditional might_sleep() if @gfp allows blocking.
 */

[dumbbell]

Address @bz comment and add __unused.