x86/xen: Fix Xen PV guest boot

x86/xen: Fix Xen PV guest boot

A recent patch moving the call of sparse_init() to common mm code
broke booting as a Xen PV guest.

Reason is that the Xen PV specific boot code relied on struct page area
being accessible rather early, but this changed by the move of the call
of sparse_init().

Fortunately the fix is rather easy: there is a static branch available
indicating whether struct page contents are usable by Xen. This static
branch just needs to be tested in some places for avoiding the access
of struct page.

Fixes: 4267739cabb8 ("arch, mm: consolidate initialization of SPARSE memory model")
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260214135035.119357-1-jgross@suse.com>