mm: do_fault(): extract to call vm_ops->do_fault() to separate function

mm: do_fault(): extract to call vm_ops->do_fault() to separate function

Extract code to vm_ops->do_fault() and basic error handling to separate
function.  The code will be reused.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>