uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT

uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT

Use the newly added FOLL_SPLIT_PMD in uprobe.  This preserves the huge
page when the uprobe is enabled.  When the uprobe is disabled, newer
instances of the same application could still benefit from huge page.

For the next step, we will enable khugepaged to regroup the pmd, so that
existing instances of the application could also benefit from huge page
after the uprobe is disabled.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Song Liu <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Srikar Dronamraju <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>