Revert "linuxkpi: GFP_KERNEL equals M_NOWAIT now"

This change seems to break some drivers such as the mlx5*(4) drivers.

As kib@ says:

According to the 'official' Linux kernel documentation, the GFP_KERNEL
flag implies sleepable context.

It was introduced while working on the new vt(4)/DRM integration [1].
During this work, doing sleepable allocations broke vt(4) and the DRM
drivers. However, I made further improvements and some locking-related
fixed to the new integration without revisiting the need for it.

After more testing, the improvements to the integration mentionned above
seems to make the change to GFP_KERNEL unneeded now. I can thus
revert it to restore expectations of other drivers.

This reverts commit 14dcd40983748596d116d91acb934a8a95ac76bc.

[1] https://github.com/freebsd/drm-kmod/pull/243

Reviewed by: kib
Approved by: kib
Differential Revision: https://reviews.freebsd.org/D42962