drm/xe/userptr: Hold notifier_lock for write on inject test path

In the Linux kernel, the following vulnerability has been resolved: drm/xe/userptr: Hold notifier_lock for write on inject test path When CONFIG_DRM_XE_USERPTR_INVAL_INJECT=y, xe_pt_svm_userptr_pre_commit() runs vma_check_userptr() with the svm notifier_lock taken for read. The test injection causes vma_check_userptr() to call xe_vma_userptr_force_invalidate(), which feeds into xe_vma_userptr_do_inval() with drm_gpusvm_ctx.in_notifier=true. That flag tells drm_gpusvm_unmap_pages() the caller already holds notifier_lock for write and only asserts the mode. Because the caller actually holds it for read, the assertion fires: WARNING: drivers/gpu/drm/drm_gpusvm.c:1669 at \ drm_gpusvm_unmap_pages+0xd4/0x130 [drm_gpusvm_helper] Call Trace: xe_vma_userptr_do_inval+0x40d/0xfd0 [xe] xe_vma_userptr_invalidate_pass1+0x3e6/0x8d0 [xe] xe_vma_userptr_force_invalidate+0xde/0x290 [xe] vma_check_userptr.constprop.0+0x1c6/0x220 [xe] xe_pt_svm_userptr_pre_commit+0x6a3/0xc60 [xe] ... xe_vm_bind_ioctl+0x3a0a/0x4480 [xe] Acquire notifier_lock for write in pre-commit when the inject Kconfig is enabled, via new helpers xe_pt_svm_userptr_notifier_lock()/_unlock(). Rename xe_svm_assert_held_read() to xe_svm_assert_held_read_or_inject_write() so it asserts the correct mode under each build configuration. Production builds (CONFIG_DRM_XE_USERPTR_INVAL_INJECT=n) keep the existing read-mode behavior bit-for-bit. (cherry picked from commit 80ccbd97ffee8ad2e73167d826fe7be548364365)