mm/huge_memory: split huge PUDs when adjusting VMA boundaries

PFN mappings such as VFIO PCI BAR mappings can use PUD leaves while
allowing VMA splits at page granularity. However, vma_adjust_trans_huge()
only handles PMDs.

After faulting a writable PUD-sized VFIO BAR mapping, changing its first
page to read-only with mprotect() makes the PMD walk treat the PUD leaf
as a PMD table. Applying MADV_DONTFORK to the first PMD-sized chunk leaves
the PUD spanning two VMAs: the PMD-aligned boundary skips splitting.

Walk the page tables once at each adjusted boundary and split a huge
PUD or PMD leaf as needed. Handle PUD leaves even when the boundary is
PMD-aligned.

Fixes: f9e54c3a2f5b7 ("vfio/pci: implement huge_fault support")
Cc: stable@vger.kernel.org
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Link: https://patch.msgid.link/20261031-fixes_core-v3-43-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

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