mm/hugetlb: Report -EHWPOISON not -EFAULT when FOLL_HWPOISON is specified

KVM uses get_user_pages() to resolve its stage2 faults. KVM sets the
FOLL_HWPOISON flag causing faultin_page() to return -EHWPOISON when it
finds a VM_FAULT_HWPOISON. KVM handles these hwpoison pages as a special
case.

When huge pages are involved, this doesn't work so well. get_user_pages()
calls follow_hugetlb_page(), which stops early if it receives
VM_FAULT_HWPOISON from hugetlb_fault(), eventually returning -EFAULT to
the caller. The step to map this to -EHWPOISON based on the FOLL_ flags
is missing. The hwpoison special case is skipped, and -EFAULT is returned
to user-space, causing Qemu or kvmtool to exit.

Instead, move this VM_FAULT_ to errno mapping code into a header file
and use it from faultin_page() and follow_hugetlb_page().

With this, KVM works as expected.

Signed-off-by: James Morse <james.morse@arm.com>