s390/gmap: improve THP splitting

During s390_enable_sie(), we need to take care of splitting all qemu user
process THP mappings. This is currently done with follow_page(FOLL_SPLIT),
by simply iterating over all vma ranges, with PAGE_SIZE increment.

This logic is sub-optimal and can result in a lot of unnecessary overhead,
especially when using qemu and ASAN with large shadow map. Ilya reported
significant system slow-down with one CPU busy for a long time and overall
unresponsiveness.

Fix this by using walk_page_vma() and directly calling split_huge_pmd()
only for present pmds, which greatly reduces overhead.

Cc: <[email protected]> # v5.4+
Reported-by: Ilya Leoshkevich <[email protected]>
Tested-by: Ilya Leoshkevich <[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Signed-off-by: Gerald Schaefer <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>

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